General Knowledge curriculum

The SI rests on seven base units: meter (length, m), kilogram (mass, kg), second (time, s), ampere (electric current, A), kelvin (temperature, K), mole (amount of substance, mol) and candela (luminous intensity, cd). All other SI units are built from these. The kilogram, not the gram, is the base unit of mass, so mass prefixes attach to gram (mg, never microkilogram).

On May 20, 2019, four base units (kilogram, kelvin, ampere, mole) were redefined by fixing exact values of constants of nature; the second, meter and candela already rested on constants. Now seven fixed constants, among them the speed of light and the Planck constant, define all seven units, ending the last artifact standard, the kilogram cylinder near Paris. Shoppers notice no change.

The kilogram is defined by fixing the Planck constant at exactly 6.62607015 x 10^-34 joule-seconds, with the meter and second already fixed. Until May 20, 2019, it was the mass of a platinum-iridium cylinder nicknamed Le Grand K, kept near Paris. Labs now realize the unit with devices such as the Kibble balance, which measures mass through electrical measurements.

Since 1967 the second has been defined by the cesium-133 atom: it equals 9,192,631,770 cycles of the microwave radiation tied to the atom's hyperfine transition, and atomic clocks simply count them. Before that, a second was 1/86,400 of an Earth day, an unsatisfactory standard because Earth's rotation speed varies slightly.

The meter is defined by fixing the speed of light in vacuum at exactly 299,792,458 meters per second, so one meter is the distance light travels in 1/299,792,458 of a second (the 1983 form). It began in revolutionary France as one ten-millionth of the distance from the North Pole to the equator; length is now derived from time and the speed of light, not the reverse.

A mole is a fixed count, not a mass: one mole contains exactly 6.02214076 x 10^23 elementary entities, which can be atoms, molecules, ions, electrons or other specified particles. How much a mole weighs depends on the substance; a mole of water molecules is about 18 grams, which is why 18 grams of water holds about 600 billion trillion molecules.

The kelvin, the SI unit of thermodynamic temperature, is defined by fixing the Boltzmann constant at exactly 1.380649 x 10^-23 joules per kelvin, which ties temperature to energy. Until 2019 it was 1/273.16 of the temperature of water's triple point. Say 'kelvin', not 'degrees kelvin': the unit takes no degree sign.

The ampere is defined by fixing the elementary charge, the charge on a proton, at exactly 1.602176634 x 10^-19 coulombs, and a coulomb is an ampere-second, so one ampere is a flow of one coulomb per second. Until 2019 the definition rested on the magnetic force between two parallel wires carrying current.

The candela measures luminous intensity: how bright a light source looks to the human eye in a given direction. Its definition fixes 683 lumens per watt for green light at 540 terahertz, a color our daytime vision favors, so light is weighted by eye sensitivity. Bulbs are sold by lumens (total light output), not candelas.

SI prefixes scale a unit by powers of ten, and 24 are recognized. In 2022 four more were added: ronna (R, 10^27) and quetta (Q, 10^30) above yotta, and ronto (r, 10^-27) and quecto (q, 10^-30) below yocto. Symbols for multiples are capitals except k, h and da, while submultiples are lowercase, so M is mega (a million) and m is milli (a thousandth).

Derived units are built from base units. The newton (N, force) is kg m/s^2; the pascal (Pa, pressure) is N/m^2; the joule (J, energy) is N m, or kg m^2/s^2; the watt (W, power) is J/s, or kg m^2/s^3; the hertz (Hz) is 1/s, used only for periodic phenomena. Special names exist for convenience, and 22 derived units have them.

When spelled out, unit names are common nouns and stay lowercase even when they honor a person (pascal, volt, weber, kelvin), but the symbol takes a capital letter (Pa, V, Wb, K). One case looks like an exception: in 'degree Celsius' the C is capital because Celsius is a person's name modifying 'degree'. Units not named for people use lowercase symbols (m, s, lm).

The speed of light in vacuum, c, is exactly 299,792,458 meters per second, a value fixed by definition since 1983 rather than measured, because the meter is now defined from it. That is about 300,000 km per second. The fixed value applies to a vacuum; light travels slightly slower through water than through air or empty space.

Absolute zero is 0 kelvin, which equals -273.15 degrees Celsius (-459.67 degrees Fahrenheit). The Kelvin scale starts there because thermodynamic temperature tracks how much average motion energy atoms and molecules have. Classical physics said all motion stops; quantum theory leaves zero-point motion, which is not heat-driven.

A degree Celsius and a kelvin are the same size, so a 10-degree rise is also 10 kelvins; only the zero point differs. Celsius temperature is kelvin temperature minus 273.15, so 0 degrees C is 273.15 K and a 21 C room is about 294 K. The degree Celsius is simply another name for the kelvin when expressing Celsius temperatures.

The claim that only the US, Liberia and Myanmar avoid metric is a myth: every country has adopted the SI, and US customary units are themselves defined in SI terms. The US legalized metric in 1866 and made it preferred for trade in 1988. Traps remain: a US gallon is 3.785 L, an Imperial gallon 4.546 L, and a force-unit mix-up doomed NASA's Mars Climate Orbiter in 1999.

Thinking in orders of magnitude means asking how many factors of ten a figure could be off, which is enough to catch impossible claims. A classic Fermi estimate puts Chicago at about 150 piano tuners; it may miss by a factor of 2 or 3, yet 15 or 1,500 would be implausible. Metric steps are tens too: a centimeter is a hundredth of a meter.

Significant figures are the digits needed to express a measured value; a result should not show more than the measurement supports. Converting 36 feet gives 10.9728 m on a calculator, but 11.0 m is the honest answer, since extra digits would claim precision the original lacked. Trailing zeros can mislead: in 1200 m it is unclear which zeros count, but 1.200 km shows four significant digits.

Scientific notation writes a number as a decimal times a power of ten: 6.0 x 10^6 is 6,000,000, and a negative exponent moves the decimal point left, so 2.5 x 10^-3 is 0.0025. It keeps huge and tiny quantities compact and avoids word ambiguity, since a billion means 10^9 in the US but 10^12 in many other countries.

A light-year is a unit of distance, not time: how far light travels in a year, about 5.88 trillion miles (9.46 trillion kilometers). Light covers 186,000 miles (300,000 km) every second, so the Sun is about eight light-minutes away, while Proxima Centauri, the nearest neighboring star, is roughly 4.25 light-years away.

The astronomical unit (au) is a fixed length of exactly 149,597,870,700 meters, set by the International Astronomical Union in 2012 to match closely the average Earth-Sun distance, about 150 million kilometers. The real distance varies and is not an exact quantity, so fixing the unit in meters simplifies solar-system calculations. The IAU writes it in lowercase, au.

The decibel scale is logarithmic: each 10 dB added makes a sound ten times more intense, yet it seems only about twice as loud. Decibels are named for Alexander Graham Bell. A-weighted decibels (dBA) also reflect how the ear responds, which is why hearing guidance uses them: 85 dBA for at least 8 hours can damage hearing.

Earthquake magnitude is logarithmic: each whole-number step is a tenfold bigger seismogram amplitude and about 32 times more energy. Richter's 1935 scale was built for California quakes; moment magnitude (Mw), based on rock rigidity, fault area and slip, is now more accurate for all but small local quakes. One quake has one magnitude but many shaking intensities.

pH measures how acidic or basic water is, on a scale of 0 to 14 with 7 neutral. It is logarithmic: each whole number is a 10-fold change, so water at pH 5 is ten times as acidic as water at pH 6, and coal-mine drainage at pH 2 is 100,000 times as acidic as neutral water. Normal rain is slightly acidic, about pH 5.6, because of carbon dioxide.

A small calorie is roughly the energy that warms 1 gram of water by 1 degree C, exactly 4.184 joules in the thermochemical definition; a kilocalorie is 1,000 of them. The 'Calories' on food labels are kilocalories, with the 'kilo' usually dropped, so a 200-Calorie snack carries 200 kcal.

Newton's three laws of motion. First (inertia): with no net force, a body at rest stays at rest and a moving one keeps its velocity; force is needed to change motion, not to sustain it. Second: acceleration depends on force and mass, F = m a, so one force speeds up a heavy object less than a light one. Third: if A pushes on B, B pushes back on A equally and oppositely, and the pair acts on two different objects.

Gravity attracts every pair of masses. The pull grows with the product of the masses and shrinks as the square of the separation between their centers, F = G M m / r^2, so doubling the distance leaves one fourth of the force. Newton's insight was universality: the force that drops an apple also holds the Moon in orbit. Heavier objects do not fall faster: apart from air resistance, heavy and light objects fall at the same rate.

Energy is never created or destroyed, only moved between objects or changed in form. Lifting an object stores potential energy equal to the work done, and letting go turns it into motion. This is the first law of thermodynamics. Conserved is not the same as reusable: entropy tracks the energy that can no longer be put to work.

Zeroth: if two objects are each in thermal equilibrium with a third, they are in equilibrium with one another, so temperature is well defined. First: energy is conserved; internal energy changes by heat added minus work done by the system. Second: a system's entropy together with that of its surroundings cannot fall, so heat moves from hot to cold on its own. Third: absolute zero is unreachable in any finite number of steps.

Entropy measures how much of a system's energy has become unavailable for useful work. The second law says the combined entropy of a system and its surroundings never falls: it holds steady only in an ideal reversible process and rises in every irreversible one. When heat moves from a hot object to a cold one, the hot object loses entropy and the cold one gains more, so the total rises.

Special relativity (Einstein, from 1905) rests on two ideas: light in a vacuum moves at one fixed speed for every observer, whatever the motion of observer or source, and physical laws take the same form in every non-accelerating reference frame. A consequence is the relativity of simultaneity: observers in relative motion disagree about which distant events happen together. Time dilation and E = mc^2 follow too.

General relativity describes gravity not as a force but as the curving of spacetime by mass and energy; planets, stars and even light follow the curved geometry. It explains what Newton's theory cannot, such as light bending near the Sun. Newton's law is not discarded: both theories remain useful, for example for putting satellites into orbit and reaching the Moon.

The uncertainty principle says a quantum system cannot have exact, simultaneous values for conjugate quantities such as position and momentum: the spreads in the two cannot both be made arbitrarily small. Kennard proved the exact form in 1927 (the two spreads multiply to at least h-bar over 2). As usually stated, it limits how sharply a state can be prepared, not instrument quality or disturbance by measuring.

Light and matter show both wave and particle behavior; which one appears depends on how they are observed. Light arrives in discrete packets called photons, whose energy is tied to frequency, yet it also diffracts into a spectrum like a wave. Electrons are particles that can also be treated as waves, described by a wave function that gives the probabilities of finding them.

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are one phenomenon, electromagnetic radiation, ordered by wavelength. From long to short wavelength, energy rises, so radio waves are the longest and gamma rays carry the most energy. Visible light is only a thin slice of the spectrum, all of it counts as light in the physical sense, and none of it needs a medium to travel.

The periodic table arranges the elements by increasing atomic number (proton count). Rows are periods; columns are groups, families with similar chemical behavior, and in the main groups, members of a column share the same number of valence (outer-shell) electrons. Properties repeat somewhat regularly, and across a period elements shift from metallic to nonmetallic. The far-right noble gases are very unreactive.

An atom has a positive nucleus of protons and neutrons with electrons around it. The proton count, the atomic number, identifies the element. Neutrons are neutral; atoms of one element with different neutron counts are isotopes, such as carbon-12 (6 neutrons, stable) and carbon-14 (8 neutrons, radioactive). Chemistry involves only the electrons, mainly the outermost ones.

Atoms bond through their electrons. In an ionic bond, electrons move from one atom to another (sodium gives one to chlorine) and opposite charges attract, building a lattice of many ions rather than a pair of atoms. In a covalent bond, atoms share electron pairs and the bond holds specific atoms together. In a metallic bond, a metal's atoms pool their outer electrons across the whole crystal.

Ordinary matter exists as solid, liquid or gas; plasma, a mix of free electrons, ions and neutral atoms, is called a fourth phase. The phase changes are named: melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid straight to gas) and deposition (gas straight to solid). They are physical, not chemical, changes: water vapor is still H2O.

An acid gives up hydrogen ions in water (H+, which are protons), so it is a proton donor; a base is what reacts with those ions, as hydroxide (OH-) does by combining with H+ to form water. Ammonia and soaps are bases too. An acid and a base neutralize each other, forming a salt and water: HCl + NaOH gives NaCl + H2O. pH is -log[H+], so each pH step is a tenfold change.

DNA is a double helix: two strands wind around each other like a twisted ladder. Each strand has a sugar-phosphate backbone carrying one of four bases, and the strands are joined by base pairs, adenine with thymine and cytosine with guanine, so each strand is the complement of the other. The strands run antiparallel (in opposite directions) in a right-handed helix, the structure Watson and Crick described in 1953.

The central dogma describes the usual flow of genetic information: DNA is transcribed into RNA, and RNA is translated into protein. Crick's 1958 version adds that information cannot flow from protein back to nucleic acid. The RNA-to-DNA arrow does exist, though: in retroviruses such as HIV, the enzyme reverse transcriptase copies viral RNA into DNA.

Natural selection needs three things: individuals vary, some of that variation is inherited, and better-suited individuals leave more offspring. Over generations, helpful inherited traits spread. Darwin's 1859 definition: keeping favorable variations and discarding injurious ones. Only heritable change counts: mutations in egg or sperm cells can be inherited, those in body cells cannot.

Gregor Mendel's pea experiments in the 1800s gave two rules of heredity. Segregation: an individual's two alleles of a gene separate into different gametes, so each parent passes on one. Independent assortment: genes on different chromosomes are inherited independently. Incomplete dominance, multiple alleles and sex linkage still obey both rules, though they do not give the classic Mendelian ratios.

Cell theory says all living organisms are made of cells, cells are the basic building blocks of life, and every cell comes from an existing cell. Schleiden and Schwann proposed that all living things are cellular; Virchow's phrase was omnis cellula e cellula, 'every cell from a cell'. Bacteria are whole organisms made of a single cell; plants and animals are built from many.

Photosynthesis converts light energy into chemical energy. Using sunlight, plants and plankton join carbon dioxide and water into sugars such as glucose and release oxygen: CO2 + H2O + energy -> CH2O + O2 in its simplest form. Animals get energy by eating that sugar. Plants also break sugar down to power their own growth, so a plant both photosynthesizes and respires.

Cellular respiration releases the energy stored in food. In its simplest form, oxygen combines with sugar to give carbon dioxide, water and energy (CH2O + O2 -> CO2 + H2O + energy), the reverse of photosynthesis. Mitochondria generate most of a cell's usable energy, stored in a molecule called ATP. Plants and animals both respire: plants break down their own sugar, animals the sugar in what they eat.

Germ theory holds that particular diseases are caused by particular microorganisms, a view confirmed by Pasteur, Lister and Koch. Koch tied anthrax (1876) and tuberculosis (1882) to specific bacteria, when tuberculosis was widely thought hereditary. His postulates set the test: the microbe is found in sick hosts, not healthy ones, is isolated and cultured, causes the disease in a healthy host, and is recovered again.

A vaccine shows the immune system an antigen without causing the disease: a killed or weakened microbe, or only a piece of one. Adaptive immune cells recognize the antigen, and memory cells persist afterward, so on a real exposure later the body can respond faster and in greater quantity. A vaccine works by stimulating your own immune response, which is how it can protect against infection without causing the disease.

Earth's rigid outer shell, the lithosphere, is broken into a dozen or more plates that creep over the hotter, softer asthenosphere below. At divergent boundaries plates pull apart and new crust forms; at convergent boundaries one plate dives under another; at transform boundaries plates slide past each other, as along the San Andreas. Boundary interactions cause earthquakes and volcanoes and build mountain ranges.

Rock is recycled among three families: igneous rock forms when molten rock (magma) solidifies; sedimentary rock forms when weathered, eroded pieces of older rock, or remains of living things, are buried, compacted and cemented; metamorphic rock forms when heat, pressure or hot fluids change any earlier rock. No type is final: any can be weathered into sediment, metamorphosed, or melted back into magma.

The water cycle moves water among the atmosphere, the surface and the ground, driven by the Sun's energy and gravity. Sunlight evaporates liquid water, and plants release vapor by transpiration (together, evapotranspiration). Vapor condenses into clouds, and water returns as precipitation, then runs off into streams or soaks into groundwater. Snow and ice can skip the liquid stage and sublimate straight to vapor.

The carbon cycle moves carbon among reservoirs: rocks and sediments hold most of it, and the remainder sits in the ocean, the atmosphere and living things. Plants take in CO2 by photosynthesis; animals exhale it, decay releases it, and the ocean exchanges it with the air; burning fossil fuels adds long-stored carbon to the atmosphere. Earth neither gains nor loses carbon overall; it only moves.

Sunlight warms Earth's surface, which gives off heat as infrared. Greenhouse gases (water vapor, carbon dioxide, methane) absorb much of it and re-radiate in all directions, including downward, which keeps the surface near 15 C on average, over 30 degrees warmer than an airless Earth. Oxygen and nitrogen, the main gases, let infrared pass. The effect is natural and essential to life; human-made emissions strengthen it.

The Big Bang model says the universe expanded and cooled from a hot, dense state. It was not an explosion at one point in space: space itself expanded everywhere, so there is no center. Three observations support it: galaxies receding from Earth (Hubble, 1929), light-element abundances, and the cosmic microwave background, the oldest observable light, released when the universe was about 380,000 years old.

Stars form in clouds of gas and dust, then spend most of their lives on the main sequence, fusing hydrogen into helium. Mass sets the course: low-mass stars burn longer, dimmer and cooler (up to trillions of years), massive ones only millions. A low-mass star such as the Sun swells into a giant, sheds its outer layers and leaves a white dwarf; a high-mass star ends in a supernova that leaves a neutron star or black hole.

The inner four planets (Mercury, Venus, Earth, Mars) are terrestrial, with solid surfaces; the main asteroid belt lies between Mars and Jupiter. The gas giants are Jupiter and Saturn; the ice giants are Uranus and Neptune. Beyond Neptune the Kuiper Belt, a ring of icy bodies, holds Pluto and most known dwarf planets. Farther still is the Oort Cloud, thought to be a spherical shell of icy comets and never imaged directly.

The scientific method is iterative: propose a hypothesis, deduce a prediction that must hold if it is true, and test it by experiment or observation, ideally against a control group that differs only in the factor under study. A failed prediction can reject the hypothesis; a passed test supports it but does not prove it. Results earn trust when other scientists can repeat them, and explanations stay open to revision.

Names mark credit, not always first discovery. Boyle's law (gas pressure and volume) honors Robert Boyle, who first observed the relation in 1660; the volume-temperature law honors both Charles and Gay-Lussac. Kepler described planetary motion, but Newton showed it follows from his laws of motion and gravitation, and Cavendish measured Newton's constant G. Kennard gave Heisenberg's uncertainty relation its exact form in 1927.

The Nobel Prizes are awarded in six fields: physics, chemistry, physiology or medicine, literature, economics, and the promotion of peace. Mathematics is not one of the six, so there is no Nobel Prize in mathematics. As an example from the science fields, Melvin Calvin won the 1961 chemistry prize for uncovering how photosynthesis turns carbon dioxide into energy-rich compounds.

Around 12,000 years ago, people in the Fertile Crescent began domesticating plants; farming later took hold in China, Mesoamerica and the Andes, bringing settled villages. The classic story says farm surplus produced rulers and hierarchy; an analysis of 868 cultures found grain growing most likely preceded states, yet intensive farming was as likely a result of states as a cause. The pace of change is debated.

Sumerians developed cuneiform, wedge-shaped signs pressed into clay tablets, around 3100 BC, to keep records of goods and payments, and few people mastered it. It is a script, not a language: tablets from the city of Ebla use it for both Sumerian and Eblaite. Hammurabi of Babylon (1700s BC) is known for one of the earliest law codes, which set penalties by the victim's rank, not a flat eye for an eye.

Egypt was united around 3100 BC under Menes; its history is divided into 30 pharaonic dynasties. Farming followed the Nile's summer flood, which peaked in August-September, with sowing after the water fell. The Rosetta Stone (found 1799) repeats one decree in three scripts; Young and then Champollion (from 1822) used it to show that hieroglyphs also spell sounds, not just pictures.

Greece was a patchwork of small city-states like Athens and Sparta, divided by mountains and sea, not one country. In Greek democracies citizens shared in public affairs, but slaves and freedmen did not. Greek philosophy began in the sixth century BC and runs through Socrates (executed 399 BC), Plato and Aristotle; tragedy and comedy grew out of festivals honoring Dionysus.

The Republic was run by a Senate of prominent men and a popular assembly that lost real power. Paid armies loyal to their generals, and civil wars, let strongmen override the Senate; Julius Caesar became dictator and was assassinated in the Senate. His heir Octavian won at Actium (31 BC) and, as Augustus from 27 BC, became the first emperor, avoiding the title king. Caesar was never emperor.

The western empire ended in 476, when the barbarian general Odoacer set aside the last western emperor, Romulus Augustulus, and told Constantinople the West had no emperor. Frontier invasions, economic strain, falling population and emperors dependent on their legions all contributed. The Greek-speaking eastern empire survived until the Ottomans took Constantinople in 1453.

Under the Abbasid caliphs, Baghdad saw the House of Wisdom and a systematic movement to translate Greek, Persian and Indian works into Arabic, which scholars then extended. Al-Khwarizmi's book on al-jabr (about 830) gave algebra its name, and Ibn al-Haytham argued in the 11th century that vision comes from light entering the eye, not rays leaving it. Latin translations carried this work to Europe in the 12th and 13th centuries.

With no strong central rule, a man in early medieval Europe traded military service and dues to a lord for protection, and lords owed loyalty to greater lords. On a manor, villeins worked the lord's own land (the demesne) as well as holdings of their own. The Church claimed a tenth of lay property, ran courts for clergy, wills and marriages, and fought kings over who named bishops.

In the mid-14th century plague hit North Africa, the Near East and Europe, where an estimated quarter to a third of the people died. With laborers scarce, wages rose, villeins left their holdings and English lords leased out demesne land; a 1351 English statute tried to hold pay at old levels, and the manorial system weakened. Plague, caused by Yersinia pestis, is curable with antibiotics.

In Renaissance Italy the leading intellectual movement was humanism: scholars treated Greek and Latin classics as guides to a moral, effective life and as models of Latin style, and studied them with new rigor. Patrons paid for it: city republics like Florence and Venice, ruling families in Milan, Ferrara and Urbino, and most popes hired humanists. The revival of learning had begun earlier, in the 12th century.

Johann Gutenberg (about 1400-1468) is generally credited with making uniform, interchangeable metal type; his Latin Bible, probably finished late in 1455 at Mainz, was Western Europe's first major book printed with movable metal type. He did not invent printing: paper and block printing came from China. Printing paid only once paper grew cheap and plentiful, and then Bibles became abundant and schoolbooks cheaper.

Voyages guided by the compass sought trade: Columbus sailed west in 1492 hoping to reach Asia, Vasco da Gama reached India around Africa in 1497-98, and one of Magellan's ships circled the globe by 1522. The Columbian exchange then moved crops, animals and microbes across the Atlantic: potatoes, maize and cassava fed Old World population growth, while smallpox devastated Indigenous populations in Central and South America.

Martin Luther challenged orthodox doctrines and practices at Wittenberg in 1517, and the printing press spread his writings in German to ordinary people. Many rulers saw advantage in breaking with Rome, and under the Peace of Augsburg (1555) German princes chose their lands' religion. The Catholic Church also revived, notably through the Jesuits.

Copernicus set out his Sun-centered model in 1543, and it took over a century to win wide acceptance. Galileo's 1610 telescope sightings of Jupiter's moons and the phases of Venus supported it, Kepler showed that planets move in ellipses, and Newton's 1687 Principia explained their motion with the laws of motion and gravity. Francis Bacon urged observation and experiment, and the Royal Society formed in 1662.

Enlightenment writers of the 17th and 18th centuries questioned absolute monarchy and argued over natural rights and limits on power. Locke held that dividing legislative and executive power protects liberty; Montesquieu's The Spirit of the Laws (1748) named legislative, executive and judicial powers, and the framers' separation of powers is credited to him. Such works guided the founding generation.

Industrialization transformed English life after about 1750, led by cotton textiles. Hargreaves's spinning jenny (1767) and, two years later, Arkwright's water frame shifted spinning toward factories powered by water and, after the 1780s, steam, which let mills rise in towns. Cotton output rose fifty-fold from 1780 to 1840, the first railway opened in 1825, and mill towns saw degraded conditions and unrest.

After 1763, British taxes and trade laws sparked protest, from the Stamp Act to the 1773 Boston Tea Party; the 1774 Intolerable Acts brought a Continental Congress. Colonial leaders still hoped to reconcile, yet on July 4, 1776, Congress adopted the Declaration of Independence. France allied with the colonies in 1778, Britain accepted independence in 1783, and the Articles of Confederation gave way to the Constitution (1789).

A bankrupt French monarchy called the Estates-General in 1789; the Third Estate turned it into a National Assembly, the Bastille was stormed, and tax exemptions, serfdom and noble privileges ended. Louis XVI was executed in January 1793, and the Terror, the most violent phase, ended in July 1794. Napoleon became First Consul in 1799 and emperor in 1804; his 1804 Civil Code became a foundation for modern civil law.

Legal slavery ended at different times. A slave revolt begun in 1791 led Haiti to independence in 1804. Britain's abolition took effect in 1834, with apprenticeship until 1838; the 13th Amendment ended U.S. slavery in December 1865; Brazil followed with the Golden Law of May 13, 1888. The 1863 Emancipation Proclamation freed people only in areas in rebellion, not nationwide.

Italy and Germany each unified around a leading monarchy. The Kingdom of Italy formed in 1861 under the House of Savoy and added Venetia and Rome by 1871. Prussia led the German states, once more than 300 independent units; after beating France, the German Empire of 1871 made the Prussian king emperor, left Austria out and took Alsace and Lorraine.

Rival alliances (France with Russia, Germany with Austria; Britain drawn toward the French side by Germany's navy) set the stage for war in 1914. Offensives stalled and armies dug into long trench lines, where advances cost enormous losses. The U.S. entered in April 1917, and fighting ended in November 1918. Versailles (1919) took land from Germany and made it accept responsibility for the war.

Wartime hardship toppled the tsar in March 1917, when Petrograd food riots grew into an uprising, and a Provisional Government took over. In November 1917 (called the October Revolution) the Bolsheviks under Lenin seized power and soon took Russia out of the war by treaty at Brest-Litovsk. A civil war between Bolsheviks and White forces, with Allied intervention, followed, and famine struck in 1921.

The 1929 U.S. stock market crash, plus trouble in Germany, France and Britain, caused a global financial crisis; scholars debate the origins, which include World War I's shock, the inflexible gold standard and weak international coordination. The 1930 Smoot-Hawley tariff entrenched protectionism, though its effects are disputed. U.S. responses included a 1933 bank holiday, the FDIC (1933) and the SEC (1934).

Fighting began in Europe in September 1939 (China and Japan had fought since 1937). Japan's attack on Pearl Harbor (December 7, 1941) drew in the U.S., and Germany then declared war on it. D-Day (June 6, 1944) landed Allied troops in Normandy, Germany surrendered May 8, 1945, and the U.S. used atomic bombs on Hiroshima and Nagasaki to end the war with Japan. The UN, IMF, World Bank and a divided Germany shaped the postwar order.

Between 1945 and 1960, about three dozen states in Asia and Africa gained autonomy or independence from European rule. There was no single path: some transitions were peaceful, others followed long wars, as in Indonesia and Vietnam. Colonial rule had drawn borders across ethnic and language groups, and Cold War rivalry shaped outcomes. UN membership grew from 35 in 1946 to 127 by 1970.

The Cold War (1947-1989) pitted the U.S. against the Soviet Union. U.S. policy rested on Kennan's 1947 containment strategy and included the Truman Doctrine (aid to Greece and Turkey), the Marshall Plan (over $12 billion) and NATO (1949); the Soviets formed the Warsaw Pact (1955). Both sides courted new states; the 1962 Cuban Missile Crisis came closest to nuclear conflict, and SALT I (1972) first capped nuclear missiles.

In Brown v. Board of Education (1954) the Supreme Court unanimously held school segregation unconstitutional, overruling "separate but equal" for public schools, though resistance was strong. Mass protest led by figures like Martin Luther King Jr. built pressure for the Civil Rights Act of 1964 (public places, schools, jobs); violence against marchers at Selma then helped bring the Voting Rights Act of 1965, which banned literacy tests.

The Soviet Union launched Sputnik, the first artificial satellite, on October 4, 1957. It carried only a radio transmitter, but its rocket was built as an intercontinental missile, so it stoked fears of falling behind, and Congress created NASA in 1958. The rivalry put animals and humans in space and led to the Apollo 11 Moon landing (July 20, 1969), meeting Kennedy's 1961 goal.

East Germany built the Berlin Wall in August 1961, first as barbed wire overnight, to stop people leaving the communist sector. It opened on November 9, 1989 after an unprepared spokesman said new travel rules covered Berlin, amid peaceful revolutions that Gorbachev's end to Soviet military intervention helped allow. Germany reunified in 1990; the Soviet Union dissolved in December 1991.

Westphalia (1648) ended the Thirty Years' War, letting German princes keep choosing their lands' religion; whether it created modern sovereignty is debated. Utrecht (1713) settled the Spanish succession: a Bourbon kept Spain if the crowns never united, and Britain won Gibraltar, Minorca and Newfoundland. Vienna (1814-15) redrew Europe after Napoleon. Versailles (1919) took German land and made Germany accept responsibility for the war.

Separation of powers splits the three basic jobs of government, making law, carrying it out and judging disputes, among different branches so that power cannot pile up in one hand. The branches also check one another: the US President can veto bills, Congress can override a veto by supermajority and impeach officials, and courts review the acts of both. The Constitution never names the doctrine, and the branches overlap by design.

In a parliamentary system the executive grows out of the legislature: the prime minister governs only with majority support, and a successful no-confidence vote brings the government down. In a presidential system such as the US, the president is elected separately, leads both state and government, and cannot be removed by such a vote. France is a hybrid: its president is directly elected, while its prime minister answers to parliament.

In a federal system the constitution splits power between national and constituent governments; the US Tenth Amendment reserves undelegated powers for the states and the people. In a unitary state the center holds final authority: China's provinces lack constitutions and Beijing appoints their leaders. Devolution delegates: Scotland's Parliament controls delegated areas, yet analysts say an independence referendum needs UK government consent.

A constitutional monarchy keeps a monarch as head of state, typically with limited or ceremonial powers. In the parliamentary versions, such as the United Kingdom, a prime minister is head of government; the monarch may formally appoint that prime minister, but the choice follows election results and often needs parliament's approval. Spain, Sweden, Norway and Denmark are other examples.

In a direct democracy citizens decide questions themselves, in assemblies or by referendum; in a representative democracy they elect lawmakers to decide for them. Madison, in Federalist No. 10, called the first a pure democracy and the second a republic, naming delegation to a small number of elected citizens as one of two great differences. Systems can mix the two: in some US states, voters can make law through a referendum.

First-past-the-post, or plurality, voting gives each voter one choice, and whoever gets the most votes wins even without a majority; most US officials are chosen this way. Proportional representation, often used in parliamentary elections, allots seats to parties by their share of the total vote. Ranked-choice voting lets voters order candidates; if none has a majority, last place is eliminated and those ballots move to their next choices.

Judicial review is the power of courts to declare that a law or government action violates the constitution and cannot stand. The US Constitution never states this power; the Supreme Court claimed it in Marbury v. Madison (1803), ruling that part of the 1789 Judiciary Act stretched the Court's original jurisdiction past what the Constitution allows and was void. It was the first time the Court invalidated an act of Congress.

The rule of law means no one is above the law, including those who govern. In the US federal courts' formulation, everyone and every institution answers to laws that are made public, applied equally, decided by independent courts and aligned with internationally recognized human rights principles. The idea has old roots: even the King was subject to the law.

Common law grew from judges' decisions: courts are bound to follow earlier cases, with statutes added on top. Civil law starts from comprehensive legislative codes, covering criminal law too, and a judge works out the facts and applies the code. The US and UK are common law; France, Germany, Mexico and Brazil are civil law, though Louisiana and Quebec are civil-law islands in the common-law world. Many countries blend the two.

Habeas corpus, Latin for "you have the body," is a court order forcing authorities to produce a prisoner and justify the detention. Its historical core is reviewing whether executive detention is lawful. The US Constitution bars suspending the writ except during rebellion or invasion when public safety requires it, and it mentions the writ nowhere else.

The Security Council's job is keeping international peace and security, and under Article 25 UN members agree to carry out its decisions. Five permanent members hold a veto (China, France, Russia, the UK and the US); ten more are elected to staggered two-year terms by the General Assembly. A substantive decision needs nine votes, and one no vote from any permanent member blocks it. Procedural matters cannot be vetoed.

The General Assembly is the UN's plenary body, where every member state has one vote regardless of size, population or power. A two-thirds majority is required for important questions, admitting members and approving the budgets. Its resolutions on peace, security and other issues are recommendations, not binding law, though they carry political weight. It also elects the Security Council's ten nonpermanent members.

The International Court of Justice at The Hague is the UN's principal judicial body. It settles legal disputes between states, and hears a case only if the states involved have consented to its jurisdiction; its advisory opinions, requested by UN bodies, are not binding. Fifteen judges are elected to nine-year terms by the General Assembly together with the Security Council. It does not try individuals; the International Criminal Court does.

The International Criminal Court, created by the Rome Statute treaty, prosecutes individuals, not states, for aggression, genocide, war crimes and crimes against humanity. Unlike the ICJ, it is not part of the UN, though the Security Council can refer cases to it or direct it to hold off investigating for up to a year. It is a court of last resort: it steps aside when a national judiciary is already effectively handling the case.

The World Health Organization, established in 1948, is the UN specialized agency for international public health. It coordinates health work across countries, sets norms and guidelines, tracks health trends and gives member states technical support. Its World Health Assembly, with all members, sets policy and appoints the Director-General. WHO belongs to the UN system yet is legally independent, with its own constitution and budget.

The IMF, founded in 1945, works to keep the international monetary system stable: it lends to countries in economic crisis, including those with short-term balance-of-payments trouble, and monitors members' economic policies. The World Bank, also created in 1945, finances development in low- and middle-income countries, from infrastructure to poverty reduction. One steadies finances in a crisis; the other funds development.

The World Trade Organization, established in 1995 as successor to the 1947 GATT, administers trade agreements, hosts trade negotiations and runs a dispute settlement process. Its rules rest on fair competition, transparency and nondiscrimination, meaning most-favored-nation and national treatment. Disputes are between member governments, not companies, and go before international tribunals.

NATO is the alliance formed under the 1949 North Atlantic Treaty, which the United States joined as one of 12 founding members. Article 5 is the treaty's mutual-defense pledge: an armed attack on one or more members is treated as an attack on all. Each member then takes the action it deems necessary, which may but need not include armed force, so the pledge is not an automatic military response.

The European Commission is the EU's executive and, in most policy areas, the only body that can propose legislation. The Council of the EU, representing national governments, shares lawmaking power with the directly elected European Parliament in many areas. The Court of Justice rules on what EU law means, and its rulings bind. Do not mix up the Council of the EU with the European Council, the summit of national leaders.

Central bank independence insulates monetary policy from the government of the day, so it serves long-run goals rather than short-term politics. It stops a government from making the central bank finance its deficits, which has repeatedly meant high inflation, and lets policymakers look past the short term. The Fed is independent yet accountable: Congress set its goals and gets its reports, but it funds itself outside Congress's budget process.

The 1949 Geneva Conventions, four treaties, protect people caught in war: wounded and sick soldiers on land, shipwrecked, wounded and sick forces at sea, prisoners of war, and civilians. They govern how war is fought, not whether a war is lawful. Common Article 3 governs conflicts that are not between states. Grave breaches, such as willful killing and torture of protected persons, are war crimes.

Proclaimed by the UN General Assembly on December 10, 1948, without a dissenting vote, the Universal Declaration of Human Rights contains 30 articles and was drafted under Eleanor Roosevelt, chair of the UN Human Rights Commission. It is a declaration, not a treaty: Congress has called it a common standard of achievement rather than a binding agreement. Even so, treaties, national constitutions and local laws widely cite it.

Nonprofit is a legal and tax status: a US organization run only for exempt purposes, with no earnings going to private individuals, can qualify for tax exemption under section 501(c)(3). NGO is a functional label for a private organization outside government, and federal usage varies: a food-aid rule limits it to groups working locally on development, while foreign-aid reporting counts for-profit firms too. So an NGO need not be a nonprofit.

The US census counts every resident every ten years because the Constitution requires an apportionment of House seats among the states. The counts also guide how trillions of federal dollars are distributed and where electoral districts are redrawn. It is not surveillance: Title 13 bars disclosing identifying information, and answers cannot be used against anyone by an agency or court.

The US Constitution gives the President no general emergency power, so authority generally comes from Congress. Under the National Emergencies Act the President must formally declare an emergency and name the laws used, and each year it lapses unless the President issues a notice continuing it. Congress can end one by joint resolution, but a veto blocks that unless two-thirds of each chamber override. Courts can review emergency actions too.

The Greek orders are told apart at the top of the column. Doric, the oldest, has a simple rounded capital and no base; Ionic has a capital with large paired scrolls (volutes); Corinthian is the slimmest and most ornate, its bell-shaped capital dressed with two rows of acanthus leaves. Corinthian capitals carry volutes too, so the leaves are the giveaway.

Gothic builders gathered a roof's weight onto isolated piers. A rib vault is a stone skeleton whose panels can be built one at a time; pointed arches let vaults over oblong bays reach equal height; flying buttresses carry the thrust over the aisles to outer piers. Walls shrank to infill, making room for huge stained-glass windows. The pointed arch is the famous marker, not the main idea.

Renaissance painters joined close study of nature to science. Linear perspective makes objects shrink as they recede, a method credited to Brunelleschi; chiaroscuro models form with light and dark, as in Leonardo's portraits. Leonardo, Michelangelo and Raphael are the best-known masters, with Titian in Venice. Northern painters like van Eyck got realism from oil detail.

Baroque art took shape around 1600, when the Catholic Church backed realistic but dramatic art to draw people into its teaching. Typical features are dynamic poses, theatrical emotion and abrupt spotlighting against dark depth, from Caravaggio's harsh contrasts to Bernini's sculpture and Rubens's huge canvases. The style soon spread to secular subjects.

Neoclassicism revived ancient Greek and Roman models in the late 1700s and early 1800s. After the French Revolution its painters, notably David, favored themes of patriotic heroism and prized strict, clean outline and solid modeling over light and color. In architecture it gave public buildings such as the U.S. Capitol designs drawn from Greece and Rome, meant to evoke the ideals of a new republic.

Romanticism put feeling and nature ahead of formal rules. Wordsworth held that good poetry springs from powerful feeling; Gericault and Delacroix broke with David's cool classicism to paint fierce passion, with color leading drawing; Turner painted nature's moods. The sublime, scenery vast or violent enough to stir awe and terror, was a Romantic ideal; Thomas Cole glorified American wilderness.

Impressionism began in France in the 1860s. Painters such as Monet and Renoir worked outdoors to record the fleeting color of changing sunlight, using short, visible, barely blended strokes and little modeled form. Official exhibitions rejected them. Manet, their friend, kept a more structured style, and Cezanne is classed with the Post-Impressionists.

Post-Impressionism names painters who rejected Impressionism's concern with passing moments to express personal feeling or their own reading of the order beneath appearances. The core names are Cezanne, Van Gogh, Gauguin and Toulouse-Lautrec. In one still life Cezanne bends a carafe to unify the composition. The label marks a reaction, not just a later date, and it opened the way to Expressionism and abstraction.

Cubism, pioneered by Picasso and Braque shortly after 1900, breaks objects into geometric planes seen from more than one angle at once: Braque's pitcher shows its rim from above and its body in profile, keeping the flat canvas in view. It aims to paint what is known about a thing, not only what the eye sees. The name began as derision.

Surrealism took shape around Andre Breton's 1924 manifesto, which treated dream and waking reality as parts of one "surreality" and called for psychic automatism, art that expresses how thought actually works. Salvador Dali used illusions and double images, a horse that is also a woman, to blur the real and the imagined, and advised artists to draw on the images of drifting into an afternoon nap.

From the 1940s on, Abstract Expressionists put the act of painting and the viewer's experience ahead of depicting objects. Jackson Pollock's dripped canvases made the trace of bodily gesture the content, what critic Harold Rosenberg called action painting; Mark Rothko's large color fields aimed to envelop the viewer, and for him a painting was an experience, not a picture of one.

Pop Art turned popular images, such as comic strips, celebrity photos and soup cans, into fine-art subjects, using humor and irony on consumer society. It emerged in Britain in the late 1950s and boomed in the United States in the 1960s. Roy Lichtenstein painted comic strips; Andy Warhol, a former commercial illustrator, made photo-silkscreen portraits of celebrities such as Marilyn Monroe and showed Campbell's Soup Cans in 1962.

Artists first condemned photography, and Ruskin held it showed ingenuity, not genius. Its defenders replied that art begins in choice and composition, even in building what the plate lacks. By 1920 pictorial photographers said the camera was increasingly seen as an art whose light is controlled to express personal impressions; later Ansel Adams used contrasting light and dark for intensity and drama.

Sculpture is made by taking away, adding, or casting. Carving removes the superfluous material from a block, such as marble, until the intended form remains; modeling builds a figure up from wax or clay, often over a support; casting turns a model into bronze. In the lost-wax method, a layer of wax between a clay core and an outer mold is melted out and bronze is poured into the gap.

Famous works have addresses. Leonardo's Mona Lisa hangs in the Louvre after years in the French royal collection; his Ginevra de' Benci is in Washington's National Gallery of Art; Lord Elgin removed Parthenon sculptures in 1801, and a large share is now in the British Museum. Provenance, a work's recorded history of ownership, matters because an object with no credible provenance can be seized and museums use it to trace looted art.

Classical epic is the most dignified, elaborate narrative poetry: heroic tales in hexameters, with Homer's Iliad and Odyssey as the model. Its conventions include invoking the Muse at the outset (the Iliad asks a goddess to sing Achilles' anger) and plunging into the middle of the story, in medias res. Virgil's Aeneid, patterned on Homer, opens with Aeneas at sea after Troy's fall, bound for Italy, where his line leads to Rome.

Greek drama grew from the worship of Dionysus, and tragedy was staged at his festival. Aristotle defined tragedy as imitation of a serious action that stirs pity and fear. Aeschylus added a second actor and trimmed the chorus; Sophocles added a third. Old comedy freely mocked real people. The three unities were codified by later French critics: Aristotle required only unity of action.

The 1623 First Folio gathered thirty-six plays under three headings: comedies, histories and tragedies. Hamlet, Othello, King Lear and Macbeth stand among its tragedies, Richard III and Henry V among the histories, A Midsummer Night's Dream and Twelfth Night among the comedies; it even lists Cymbeline as a tragedy. The 154 sonnets (1609) typically rhyme alternately for twelve lines, then close with a couplet.

The English novel is customarily dated from Richardson's Pamela (1740), a break from the long romances before it. Fielding mocked it in Joseph Andrews (1742), imitating Cervantes, and gave the form its first accurate picture of contemporary manners. In France Balzac built his novels into one structure, the Comedie humaine; in Russia Dostoevsky's Crime and Punishment carried psychological study further, and Tolstoy followed.

Realism, in Howells's words, is the truthful treatment of material; naturalism, realism in its advanced stage under Zola, casts the novelist as a scientific observer who dissects life. Modernism, as Woolf argued of Joyce, turns inward to capture a mind's passing impressions rather than follow plot. Postmodern fiction, as in Barth's metafiction, plays with intertextual links and, critics say, moves away from depth.

A sonnet has fourteen lines: an eight-line octave plus a six-line sestet in the standard form, while Shakespeare's alternates rhymes for twelve lines and closes with a couplet. A villanelle is a poem in tercets with only two rhymes, its first and third lines recurring as refrains. A haiku is a brief Japanese form of 5-7-5 syllables. Free verse drops fixed meter but keeps a marked cadence, closer knit than prose.

A metaphor states that one thing is another ("All the world's a stage"); a simile says it is like another. Metonymy swaps a word for something associated with it, as in "we read Virgil" for his poems; synecdoche is the part-for-whole case, like "fifty sail" standing for fifty ships. Verbal irony says the opposite of what is meant; an allusion is an indirect reference to something assumed to be known.

Alfred Nobel's will set up prizes in physics, chemistry, physiology or medicine, literature, and peace; peace is presented in Oslo, the rest in Stockholm. Economics was added later, endowed in 1968 by Sweden's central bank. Mathematics has no Nobel; Norway's Abel Prize (2001) is its counterpart. Famous omissions, each nominated five times: Tolstoy (literature) and Gandhi (peace).

'Classical music' broadly covers several eras; the Classical era is only one. Baroque (about 1600-1750), as in Bach, is polyphonic and contrapuntal. Classical (about 1730-1820), the style of Haydn and Mozart, favors clarity and balance, with a melody over chordal accompaniment. Romantic (about 1815-1910), with Chopin and Liszt, expands form and harmony; Modern (from about 1900) stretches or abandons tonality.

An orchestra's instruments fall into four groups: strings (violins, violas, cellos, double basses), woodwinds (flutes, oboes, clarinets, bassoons), brass (trumpets, horns, trombones, tuba) and percussion (drums, cymbals, triangle). The usual plan seats strings in front with first violins on the conductor's left, woodwinds in the middle, and the loudest brass and percussion at the back; conductors adjust it.

Pitches are written on a five-line staff, and a clef fixes which pitches the lines stand for: the G (treble) clef makes the second line G, the F (bass) clef makes the fourth line F. The key signature, the sharps or flats at the start, partly shows the key, and one signature fits a major key and its relative minor. A time signature like 3/4 means three quarter-note beats per measure, the first accented.

The major scale has half steps between degrees 3-4 and 7-8 and whole steps elsewhere; minor has a smaller third (three half steps, not four) in natural, harmonic and melodic forms. Church modes use natural notes from different home tones: Dorian on D, Phrygian on E, Lydian on F, Mixolydian on G; Ionian matches major, Aeolian natural minor. The names were borrowed from Greek scales through a medieval misreading.

Baroque names are Bach (preludes and fugues in every key) and Handel (oratorios like Messiah, after giving up opera). Haydn, called father of the symphony and string quartet though he had predecessors, and Mozart, known for operas and symphonies, define the Classical era; Beethoven enlarged sonata form toward Romantic expression. Romantics include Schubert (songs), Chopin (piano) and Wagner (music dramas).

Opera is drama set to music: the libretto is its text, the overture opens it, recitative is sung speech and an aria an elaborate song for one voice. It began in Florence in the 1590s; staples include Mozart's Don Giovanni, Verdi's Aida and Wagner's Ring. Ballet tells a story through dance, gesture and music, as in Tchaikovsky's Swan Lake, The Sleeping Beauty and The Nutcracker.

A shot is the continuous stretch of film between two cuts, and a cut is the edit that moves from one shot to the next. Classic montage joins shots so their pairing makes new meaning, as in Lev Kuleshov's 1920s experiment pairing one neutral face with different scenes. Mise-en-scene ('setting the stage') is the staging in front of the camera: performers, set, props, costumes, lighting.

A sign language is a full natural language with its own grammar, not a gestured version of speech. American Sign Language is distinct from English: it is signed with the hands and face, and a question is marked with raised eyebrows and widened eyes. No single sign language is used worldwide; British Sign Language is not the same as ASL. Fingerspelling, one handshape per letter, is only one part of ASL.

English, Latin, Greek, Persian and Sanskrit descend from one ancestral language, a family first glimpsed in 1786 when Sir William Jones saw that Sanskrit shares a source with Greek and Latin. Related words show it: hundred is Latin centum, Greek hekaton and Sanskrit satam. Sanskrit is a sister of the rest, not their mother, and Basque and the Dravidian languages of India are not members.

Sino-Tibetan joins Sinitic languages such as Mandarin and Cantonese with the Tibeto-Burman languages of the Himalayas and Southeast Asia. Afro-Asiatic spans the Middle East and Africa (Arabic, Hebrew, Amharic, Hausa, Somali). Niger-Congo, the family with the most languages, covers most of sub-Saharan Africa; Bantu is its widest branch. Austronesian, traced to Taiwan, stretches from Indonesia to Easter Island.

Scripts differ in what each sign stands for. An alphabet gives consonants and vowels their own letters; an abjad, as for Arabic and Hebrew, writes consonants and leaves vowels optional. An abugida, as for Hindi and Thai, gives each consonant sign a built-in vowel that marks can change. A syllabary, like Japanese hiragana, has a sign per syllable, and Chinese characters stand for whole morphemes.

Cuneiform (from Latin cuneus, wedge) began as Sumerian picture-writing that turned into wedge shapes in clay, and was later used for many languages. Its decipherment began with Old Persian, one of three languages on Darius's cliff inscription at Behistun, whose Persian text Rawlinson had largely translated by 1846. Egyptian hieroglyphs yielded to Champollion in 1822, who showed that their signs spell sounds, not only ideas.

The Rosetta Stone, found in 1799 by French engineers with Napoleon's expedition to Egypt, carries one priestly decree in three scripts: hieroglyphic, demotic and Greek. Scholars could read the Greek and match names across scripts, but the hieroglyphic text was too broken to unlock the writing alone. Thomas Young made a start; Champollion's breakthrough came in 1822, with help from other inscriptions and from Coptic.

Latin entered English in waves. With Christianity came church words such as monk, nun, mass and school; after the Norman Conquest, French brought many Latin-based words; and Renaissance scholars added more, even respelling doute and dette as doubt and debt to show their Latin roots. Latin still lives in court terms such as habeas corpus (you have the body) and pro se, and in the New Latin names of species.

Greek supplies much of the building material of science and medicine, often in coinages the ancient Greeks never used. Arthritis joins Greek arthron (joint) to the ending -itis (inflammation of); photograph joins photo- (light) to -graph (write); biology joins a word for life to -logy (science). Telephone, photograph and geology are 19th-century formations from Greek parts.

After the Norman Conquest, French became the language of the English court and nobles, while English stayed with ordinary people. So the live animal kept a Germanic name (cow, swine, sheep, calf, deer) and the meat at the lord's table got a French-derived one (beef, pork, mutton, veal, venison). Walter Scott's novel Ivanhoe has a jester explain the split, and French also supplied many words for law, art and learning.

Between about 1350 and 1700, the long vowels of English moved higher in the mouth, and the two highest, ee and oo, broke into diphthongs: hus (said hoose) became house, and Middle English bite sounded roughly like modern beet. This is called the Great Vowel Shift. Printing standardized spelling meanwhile, so the letters kept older vowel values while speech moved on.

English grows by borrowing. Arabic gave algebra (from a root meaning 'to bind together'), alcohol, coffee, sugar and zero; Hindi gave shampoo (from 'to press'), pajamas and jungle; Dutch gave yacht, skipper, landscape and brandy (from 'burn' plus 'wine'); Japanese gave kimono, samurai and hara-kiri; and Nahuatl, by way of Spanish, gave chocolate and tomato.

An eponym is a word taken from a person's name. Boycott honors Captain Boycott, an estate manager in Mayo, Ireland, whom neighbors shunned in 1880; sandwich, the Earl of Sandwich; silhouette, Etienne de Silhouette, a French finance minister of 1759, though how his name came to mean a profile is disputed. The guillotine honors Dr. Guillotin, who urged abolishing beheading by ax or sword; Dr. Antoine Louis invented it.

A toponym is a word taken from a place name. Cologne is perfumed water first made in Cologne, Germany; jean cloth is probably named for Genoa; muslin comes from Mosul; damask from Damascus; cantaloupe from Cantalupo in Italy. Dollar descends from Joachimsthaler, coins first struck about 1518 in Joachimsthal, Bohemia, and cravat from a Croat, since France copied the neckcloth from Croat troops in 1636.

A portmanteau word packs two words into one. Lewis Carroll's Humpty Dumpty named the idea: slithy holds 'lithe' and 'slimy,' like a bag packed with two things. Smog (smoke + fog) was coined by a Londoner in 1905, not in Los Angeles; brunch is breakfast + lunch, motel is motor + hotel, and podcast (iPod + broadcast) dates from 2004.

An acronym is pronounced as a word, like NATO, radar, laser or scuba; an initialism is pronounced letter by letter, like FBI, CDC or DNA. Style guides disagree about which term is the umbrella, and everyday usage often calls both acronyms. Pronunciation also decides a or an: an HMO report, but a NATO ally.

Folk etymology is a popular but false account of where a word came from, or the reshaping of a word by a misunderstanding. Sirloin was not knighted by a king as Sir Loin; it is a corruption of French surloin, 'upon the loin.' Humble pie has no tie to humility: it was umble pie, made from a deer's inferior parts. Be wary of any origin tale built on a person or anecdote, and trace the word's earliest form.

The ending -ology comes from a Greek form meaning discourse or science, so it names a field of study. Etymology studies the history of words, entomology studies insects, ornithology studies birds, and oenology is knowledge of wine. Don't swap the first two: etymology is words, entomology is insects (the Greek root means cut in, since insects look nearly cut in two).

Many numbers have a Greek prefix and a Latin twin: tetra and quadri (four), penta and quinque (five), hexa and sex (six). Greek forms turn up in shape and chemical names (pentagon, hexagon, pentoxide); Latin ones in words about time and division (quinquennial, sexennial, quadrant). The metric system mixes both: the prefixes deca and kilo are Greek, while deci and centi are Latin.

Many collective nouns come from a list of proper terms for gentlemen printed in the 1486 Boke of St Albans and attributed to Dame Juliana Berners: a pride of lions, a sleuth of bears, an exaltation of larks, an unkindness of ravens. The list also joked about people, as in a superfluity of nuns. It is playful wordplay, not natural history, and this 1486 list has no murder of crows.

A spelling alphabet gives each letter a code word so messages survive a poor radio or phone line: Navy is spoken November Alfa Victor Yankee, and the point is to keep sound-alikes such as m and n apart. The international list, often called the NATO alphabet, was designed by the aviation body ICAO for speakers of many languages. The U.S. Navy adopted it in 1956, replacing wartime words such as Able, Baker, Charlie.

Braille and Morse code turn letters into patterns another sense can read. Louis Braille had developed his code by age 15: each letter is a pattern of up to six dots in a cell two dots wide and three high, simplified from a military night-writing system. Morse code, shaped in the 1830s by Samuel Morse and Alfred Vail, uses short and long signals, dots and dashes; International Morse followed in 1851.

A constructed language is designed on purpose, not grown naturally. Esperanto, published in 1887 by the physician Zamenhof, was meant as a neutral auxiliary language standing apart from national tongues. It draws roots from several European languages, builds many words from a small stock of roots (amiko is friend, malamiko enemy), and has just five vowels. An earlier planned language, Volapuk, came first.

Samuel Johnson's Dictionary of 1755 tried to fix the language, yet he soon doubted a living language can be frozen, and he showed how useful quotations are. Noah Webster's American Dictionary appeared in 1828. The Oxford English Dictionary grew from Richard Trench's 1857 call for an inventory of the language, not a judge of usage; its first part came out in 1884, with dated quotations for each word and sense.

A proverb is a short, complete saying that states a practical truth, such as 'Speech is silvern; silence is golden.' An idiom is a phrase whose meaning does not follow from its words: to kick the bucket means to die, and to break the ice means to get past first difficulties and make a beginning. The sense belongs to the whole expression, so reading it word by word misleads.

Anaphora repeats a word at the start of successive clauses. Chiasmus reverses the order of repeated terms: voice and hands, then hands and voice. Litotes understates by denying the opposite: a citizen of no mean city means an illustrious one. Zeugma, Greek for yoke, ties two words to one verb or adjective, as in Dickens's 'in a flood of tears and a sedan-chair.'

The twelve Olympians are Zeus (king of the gods; thunderbolt), Hera (queen, marriage), Poseidon (sea; trident), Demeter (agriculture), Athena (wisdom; owl; born armed from Zeus's head), Apollo (music, prophecy, light), Artemis (hunting), Ares (war), Aphrodite (love), Hephaestus (fire, the forge), Hermes (messenger; winged shoes) and Hestia (hearth). Hades, lord of the dead, and Dionysus, god of wine, stand outside the classic list.

Romans matched their gods to Greek ones: Zeus to Jupiter, Hera to Juno, Poseidon to Neptune, Athena to Minerva, Ares to Mars, Aphrodite to Venus, Hermes to Mercury, Artemis to Diana, Hephaestus to Vulcan, Demeter to Ceres, Dionysus to Bacchus. It pairs roles, not words: Apollo kept his Greek name, Janus has no Greek twin, and Mars, Rome's protector and father of Romulus and Remus, was honored where Ares was widely disliked.

Heracles (Roman Hercules) served Eurystheus through twelve labors, from the Nemean lion and Hydra to the Hesperides' golden apples and Cerberus. Perseus beheaded Medusa, whose gaze petrified onlookers, by watching her reflection in a shield. Theseus slew the Minotaur in the Cretan labyrinth and escaped by Ariadne's thread. Odysseus (Roman Ulysses), the schemer behind the wooden horse, spent twenty years away from Ithaca.

Hades names both the god and his realm. The dead cross a river (Styx, with Acheron and Cocytus) in Charon's boat, but he takes only those buried and able to pay a coin; three-headed Cerberus lets all in and none out. Judges such as Minos sort the shades: the blessed go to the Elysian Fields, the guilty are hurled into Tartarus, a pit as far below Hades as earth is from sky, where the Titans are held.

Eris's golden apple set Hera, Athena and Aphrodite quarreling; Paris judged Aphrodite fairest and she gave him Helen, wife of Menelaus, so the Greeks under Agamemnon besieged Troy. The Iliad begins in year ten with Achilles' quarrel and ends with Hector's funeral. Later poems add Achilles' death from Paris's arrow, the wooden horse, Troy's fall and the Greeks' journeys home. The vulnerable heel is a later tale, not in Homer.

Odin, the Allfather, leads the Aesir gods of Asgard; Thor, strongest of gods and men, wields the hammer Mjolnir; Loki, cunning and deceitful, fathers the Fenris wolf and the earth-encircling Midgard serpent. The nine worlds include Asgard (gods), Midgard (humans), Jotunheim (giants), icy Niflheim and fiery Muspelheim. At Ragnarok the wolf kills Odin, Thor and the serpent kill each other, and fire consumes the world before it rises anew.

In the old legends Ra, the sun-god, rules Egypt as king. Osiris, a benevolent king killed by his brother Set, is raised to life by Isis and their son Horus and becomes lord of the dead and a symbol of resurrection. Anubis helped embalm Osiris and tends the Great Scales where the dead are judged: the heart is weighed against a feather standing for truth, and the monster Ammit devours hearts found light.

Gilgamesh, king of Uruk, mourns his friend Enkidu and seeks immortality from the flood survivor Utnapishtim, but a serpent steals the plant of renewed youth he wins. In Utnapishtim's account Ea warns him to build a boat before the gods' flood, and a dove, swallow and raven are sent out afterward. The Enuma Elish, the creation epic, has Marduk kill the monster Tiamat and fashion heaven and earth from her body.

Brahma (creator), Vishnu (preserver) and Shiva (destroyer) are often grouped as a divine triad, yet followers of Vishnu and of Shiva each hold their own god supreme. Vishnu is said to take earthly forms, avatars, to rescue the world; as Rama he defeats the demon king Ravana. The usual ten are a fish, tortoise, boar, man-lion, dwarf, Parashurama, Rama, Krishna, Buddha and Kalki, still to come.

The Buddha taught the Four Noble Truths: suffering exists, craving causes it, ending craving ends it, and the Eightfold Path leads there (right views, aspiration, speech, conduct, livelihood, effort, mindfulness, meditation). Theravada, 'teaching of the elders,' keeps the Pali canon in Sri Lanka, Myanmar and Thailand; Mahayana, the 'Great Vehicle,' honors bodhisattvas who delay nirvana for others; Tibetan Buddhism is mostly tantric (Vajrayana).

Adam, Noah, Abraham, Moses, David and Solomon appear in the Bible and the Qur'an, as Adam, Nuh, Ibrahim, Musa, Dawud and Sulayman. Abraham is the shared patriarch: Genesis puts God's covenant with his son Isaac, while the Qur'an also honors Ishmael, ancestor of the Arabs. Christians and Muslims both revere Jesus (Isa) and Mary (Maryam) but differ on his nature: the Qur'an calls him a messenger and says God has no son.

The Torah is the five books of Moses; traditional Judaism holds that an oral Torah was given at Sinai too. The Talmud is the Aramaic commentary on the Mishnah, the collected oral teaching, in a Jerusalem and a Babylonian version. Passover recalls the Exodus, Shavuot (Weeks) the Ten Commandments, Sukkot the wilderness tents, Rosh Hashanah and Yom Kippur judgment and atonement, Hanukkah the Maccabean victory, Purim the rescue from Haman.

Christianity's main branches, Catholic, Orthodox and Protestant, came from two great splits. East and West drifted apart over Rome's primacy, the Western Filioque addition to the creed and customs like unleavened bread, then separated formally with mutual excommunications in 1054. Luther's Ninety-five Theses against indulgences (1517) began the Reformation; England's Act of Supremacy (1534) made the king head of its church.

The five pillars are the declaration of faith (no god but God, and Muhammad is his messenger), prayer five times daily, almsgiving (zakat), fasting in Ramadan, and pilgrimage to Mecca once in a lifetime if able. Sunnis and Shia differ over who rightfully succeeded Muhammad: Sunnis accept the first four caliphs; Shia hold that his cousin and son-in-law Ali was the rightful first successor, and revere imams as leaders.

Sikhism, founded by Guru Nanak, teaches one God and rejects caste distinctions; ten Gurus led it, and the tenth, Gobind Singh, gave his followers the name Singh and the practices of uncut hair and carrying steel. Jainism, taught by Mahavira, the 24th Jina, holds the world uncreated and without a ruling god, souls bound by karma until liberation, and nonviolence (ahimsa) as the first vow; its branches are Digambara and Svetambara.

Confucianism, from Confucius, is an ethic of humane conduct, ritual propriety and filial piety; he named reciprocity as the rule for a lifetime: do not inflict on others what you would resent yourself. Daoism, in the Tao Te Ching credited to Laozi, centers on the Dao, a Way beyond words, and on non-forcing action. Shinto, Japan's native worship of kami at shrines with purification rites, means 'the way of the gods.'

Many peoples tell of tricksters and culture heroes who shape the world. In Tlingit tellings Raven helps make the world and wins light that a rich man hoarded; in Polynesia Maui snares the sun, hauls up islands and finds fire; Cherokee tradition makes Rabbit its trickster. Origin stories differ: in Cherokee tradition the water beetle dives for mud that grows into the earth, later hung from the sky by four cords.

An archetype is an original pattern or model, and myths from far-apart places repeat a few: a flood that spares one family (Utnapishtim, Noah, Deucalion); a culture hero who wins humanity a gift, like Prometheus's fire or Raven's light; a trickster who lives by wit, like Coyote, Anansi or Brer Rabbit; and a god who dies and returns, like Osiris. Such parallels show recurring themes, not necessarily one shared source.

A myth is an ancient traditional story of gods, heroes or the origin of things that a people once took as true. A legend is a tale from the past, such as a saint's life, that records cannot verify and that is told as true, so listeners may believe or debate it. A folktale is any story passed on by word of mouth, including magic, tall and trickster tales. The lines blur, and myth does not simply mean false.

Echo, a nymph, pined for Narcissus until only her voice remained; the word echo, from Greek for sound, names that voice. Narcissus, enamored of his reflection, gave narcissism its name (coined in the 1890s). Tantalus, punished below by water and fruit that recede from his reach, gives tantalize. Panic is named for Pan, to whom causeless fright was ascribed; jovial for Jove, whose planet cheered people; mercurial for swift Mercury.

The usual count is seven continents (Africa, Antarctica, Asia, Australia, Europe, North America, South America), but borders are conventions: the Europe-Asia line is disputed, with the Urals commonly cited. There is also one global ocean, split into the Atlantic, Pacific, Indian, Arctic and Southern; most countries, including the US, count the Southern as the fifth, though its boundaries are unratified.

Area counts land and population counts people, so the rankings differ. Russia is the largest country by area (about 17.1 million square kilometers) yet ranked only seventh in population in 2006; India, seventh by area, was second in population behind China (2004). Australia, 7.7 million square kilometers but 93 percent arid, had 2.6 people per square kilometer (2005).

Capitals are chosen, not ranked by size. Canberra is Australia's capital, but Sydney is far larger (in 2005, 4.2 million people against Canberra's 323,000). Nigeria planned a move from Lagos, its most populous city, to the more central Abuja, which became the capital in December 1991.

Some states split government across cities. South Africa has three capitals: Pretoria (Tshwane) is administrative and executive, Cape Town legislative and Bloemfontein judicial. Bolivia's executive capital is La Paz and its judicial capital is Sucre. In Malaysia, Putrajaya has been the administrative capital since 1999 and holds most ministries, yet Parliament still sits in Kuala Lumpur.

A landlocked state has no ocean coastline, so its trade must cross other countries to reach the sea, which raises costs substantially (Kazakhstan is an example). Uzbekistan is double-landlocked: its neighbors are landlocked or, like Kazakhstan and Turkmenistan, touch only the landlocked Caspian Sea. It has relied heavily on transport corridors through Russia.

Length, drainage basin (the land that drains to a river) and history are separate facts. As of 2006 the Yangtze ranked behind only the Amazon and the Nile in length, with a 1.8 million sq km catchment. River valleys nurtured early civilizations: Sumer (Tigris and Euphrates), the Harappans (Indus) and Egypt, which Herodotus called a gift of the river.

Major ranges often mark plate collisions. India's crash into Asia about 50 million years ago pushed up the Himalayas and Tibetan Plateau; the Caucasus rose as the Arabian plate collided with Eurasia. The Andes rise where the Nazca Plate dives under South America. The Urals formed in continental collisions 250 to 300 million years ago, so they count among Earth's oldest ranges.

Deserts cluster near 30 degrees north and south because of Hadley cells: hot air rises in the tropics, flows poleward and sinks near 30 degrees. That sinking air is dry and warms by compression, so skies stay clear under a belt of subtropical high pressure. Deserts get about 250 mm (10 inches) of rain a year, the least of any biome, while rainforests lie between the tropics.

Time zones split the 360 degrees of longitude into 24 bands, each about 15 degrees wide and a set number of hours from UTC. Borders bend to suit local populations, and everyone in a zone shares one clock time. Some zones use odd offsets: India is one zone at UTC+5:30, Nepal is UTC+5:45 and central Australia is UTC+9:30.

The International Date Line, roughly along 180 degrees longitude in the Pacific, separates two calendar dates: cross it heading west and it is a day later. It is a convenience with no force in international law, so it zigzags around borders such as eastern Russia, and governments have moved it: Samoa switched sides at the end of 2011, skipping a day on its calendar.

The prime meridian is longitude 0, running through the Royal Observatory at Greenwich, England. Unlike the equator it is arbitrary: representatives of 25 nations chose it at the 1884 International Meridian Conference in Washington, D.C., partly because many charts and time systems already used Greenwich. A GPS reads 0 degrees about 102 meters east of the old markers.

The equator is 0 degrees latitude, splitting the planet into northern and southern halves (hemispheres). The tropics lie between the Tropic of Cancer and the Tropic of Capricorn, about 23.4 degrees north and south, matching Earth's axial tilt. They are the farthest latitudes where the Sun can stand straight overhead at noon: around June 21 at Cancer and December 21 at Capricorn.

Seasons come from Earth's axial tilt of about 23.4 degrees, not from its distance from the Sun. Around June the North Pole leans toward the Sun, giving the Northern Hemisphere more direct sunlight and summer; around December the South Pole does. Earth is actually closest to the Sun during Northern Hemisphere winter.

Moon phases come from how much of the Moon's sunlit half we see as it orbits Earth, a cycle of about 29.5 days. The Sun always lights half the Moon: at full Moon the near side is lit, at new Moon the far side is. Earth's shadow is not the cause, except in a lunar eclipse, and the far side is no permanent dark side: it receives as much sunlight as the side facing us.

A solar eclipse happens at new Moon when the Moon lines up between Sun and Earth; a lunar eclipse happens at full Moon when Earth sits between the Sun and Moon. They are not monthly since the Moon's orbit tilts about five degrees from the Earth-Sun plane, so the new Moon typically slips above or below the Sun, and only two 'eclipse seasons' a year line up.

Tides come from the gravity of the Moon and Sun on the oceans. The Moon dominates: the Sun's tide-generating force is about half as strong, and two water bulges track the Moon as Earth turns. Spring tides, the largest range, occur when the Sun and Moon line up, at new Moon and at full Moon; neap tides, the smallest, come at the quarter Moons. 'Spring' is unrelated to the season.

Plates meet in three main ways. At divergent boundaries they pull apart and new crust forms, as on the Mid-Atlantic Ridge. At convergent boundaries one plate dives under another (subduction), making trenches, volcanoes and strong earthquakes, or continents collide into mountains. At transform boundaries, like the San Andreas Fault, plates slide past each other with shallow quakes.

Shield volcanoes, like Mauna Loa, are broad, gently sloped cones of fluid basalt lava, and most eruptions are nonexplosive. Composite volcanoes (stratovolcanoes), such as Mount Fuji, are steep cones of alternating lava and ash, from effusive to extremely explosive. A caldera is a large collapse basin left when a volcano loses its top, as at Crater Lake, Oregon, about 6,800 years ago.

Winds drive surface currents, and Earth's rotation (the Coriolis effect) bends them into five major gyres, clockwise in the north and counterclockwise in the south. The Gulf Stream, a strong western boundary current, warms western Europe. Deep currents are driven by density: cold, salty polar water sinks and feeds a global conveyor belt whose loop can take about 1,000 years.

A biome is a set of plants and animals adapted to a shared climate. Tundra is the coldest, nearly treeless over permafrost; boreal forest (taiga) is a coniferous belt just south of it; temperate deciduous forests have four seasons. Yearly rain runs from about 250 mm in deserts and 500 to 900 mm in grasslands to 2,000 to 10,000 mm in rainforests.