Rölvaag, Ole Edvart (1876–1931), Norwegian-born novelist, who came to the United States in 1896 and wrote in Norwegian.
21st Century Webster's Family Encyclopedia - Respiratory system to Roman Empire
Río de la Plata, estuary formed by the Paraná and Uruguay rivers, separating Argentina and Uruguay.
Respiratory system See: Respiration.
Restaurant, food-and-drink facility that serves the public.
Restigouche River, river in northeastern Canada, more than 100 mi (160 km) long, forming part of the boundary between Quebec and New Brunswick.
Restoration, name given to the return of Charles II as king of England in 1660, after the fall of the protectorate.
Resurrection, act of God believed to restore life in perfected form to the dead.
Resurrection plant, one of several species of plants that curl up when dry but turn green when exposed to water.
Resuscitator See: Respirator.
Retailing, selling of merchandise or services to the public.
Retainer, in law, agreement between an attorney and client for legal representation.
Retardation See: Mental retardation.
Retina See: Eye.
Retriever, breed of sporting dog trained to search out and bring back small game shot by hunters.
Reunion, volcanic island covering 970 sq mi (2,512 sq km) in the West Indian Ocean.
Reuter, Baron de (Paul Julius von Reuter; 1816–99), German-born founder of Reuters, the worldwide news agency.
Reuters, one of the largest international news agencies, based in Britain, that distributes information to local agencies, newspapers, television, and radio to more than 150 countries.
Reuther, Walter Philip (1907–70), U.S. labor leader, president of the United Automobile Workers from 1946 until his death, and important labor spokesperson.
Revelation, Book of, or Apocalypse, the last book of the New Testament.
Revels, Hiram Rhodes (1827–1901), pastor, educator, and first black U.S. senator.
Revenue, internal See: Internal revenue.
Revenue sharing, return of U.S. tax revenues to the state and local governments.
Revere (pop. 42,423), resort suburb on Massachusetts Bay, north of Boston.
Revere, Paul (1735–1818), American Revolutionary hero, immortalized by the poet Henry Wadsword Longfellow for “Paul Revere's Ride” from Boston to Lexington (April 18, 1775) to warn the Massachusetts minutemen that the British were coming.
Reversing Falls of Saint John, natural wonder at the mouth of the St.
Revivalism, in religion, emphasis on personal experience and salvation of the soul.
Revolution, fundamental change in the form or nature of a government or societal way of life.
Revolution of 1848, series of unsuccessful revolutionary uprisings in France, Italy, the Austrian Empire, and Germany in 1848. Each was relatively spontaneous and self-contained, but all had a number of common causes: the successful example of the French Revolution of 1789, economic unrest due to bad harvests and unemployment, and a growing frustration, fired by nationalist fervor, about the repre…
Revolutionary War in America (1775–1783), also known as the American Revolution, in which Britain's 13 North American colonies lying along the Atlantic seaboard won their independence. It was a minor war at the time that had immense consequences later—the founding of a new nation, the United States of America. Differences in life, thought, and interests had developed between E…
Revolver, pistol with semiautomatic action made possible by the incorporation of a revolving cylinder carrying several bullets.
Rexroth, Kenneth (1905–82), U.S. poet.
Reye's syndrome, rare disease that attacks the liver and central nervous system of children age 4–15.
Reykjavik (pop. 100,800), capital of Iceland and its chief port, commercial and industrial center, and home of its cod-fishing fleet.
Reynard the Fox, leading character in a popular medieval series of fables.
Reynaud, Paul (1878–1966), French statesman.
Reynolds, Sir Joshua (1723–92), perhaps the most famous English portrait painter.
Reza Shah Pahlavi (1877–1944), shah of Iran (1925–41).
Rh factor, protein substance appearing on the surface of red blood cells of most people (85% or more), capable of inducing an immune response.
Rhône River, important European river, 507 mi (816 km) long, rising in Switzerland and flowing through Lake Geneva and then southwest and south through France into the Mediterranean Sea.
Rhea, large flightless South American bird of the order Rheaformes.
Rhea, in Greek mythology, wife and sister of Cronus (ruler of the Titans), daughter of Gaea (the earth) and Uranus (the sky).
Rhee, Syngman (1875–1965), president of South Korea.
Rhenium, chemical element, symbol Re; for physical constants see Periodic Table.
Rheostat, variable resistor used to control the current drawn by an electric motor to dim lighting.
Rhesus monkey (Maccaca mulatto), monkey found in southern and south-eastern Asia.
Rhetoric See: Oratory.
Rheumatic fever, feverish illness, following infection with streptococcus and leading to systemic disease.
Rheumatism, term popularly applied to pain affecting muscles, tendons, joints, bones, or nerves, in such widely varied disorders as rheumatoid arthritis, degenerative joint disease, spondylitis, bursitis, fibrositis, myositis, neuritis, lumbago, sciatica, and gout.
Rhine River, longest river in western Europe, rising in Switzerland and flowing 820 mi (1,320 km) through Germany and the Netherlands into the North Sea near Rotterdam.
Rhineland, region of Germany along the Rhine River and its tributaries.
Rhinitis, most frequent of the acute upper respiratory infections, characterized by edema, swelling and widening of the blood vessels of the mucous membrane of the nose, nasal discharge, and obstruction.
Rhinoceros, any of 5 species of heavy land mammals (family Rhino-cerotidae) characterized by one or two nasal “horn” or “horns,” formed of a mass of compacted hairs.
Rhizoid See: Moss.
Rhizome, or rootstock, swollen horizontal underground stem of certain plants, such as ginger.
Rhode Island, state in New England, the northeastern region of the United States; bordered by Massachusetts to the north and east, the Atlantic Ocean to the south, and Connecticut to the west. Rhode Island has two main land regions. The Coastal Lowlands cover roughly half of the state's mainland, plus the islands in Narragansett Bay and all the land to the east of the bay. Low inland hills …
Rhodes, or Ródhos, Greek island covering 540 sq mi (1,399 sq km), off the southwest coast of Turkey.
Rhodes, Cecil John (1853–1902), English politician and business magnate who first opened up Rhodesia to European settlement.
Rhodes, John Jacob (1916– ), U.S. political leader.
Rhodes Scholarship, award instituted (1902) at Oxford University by the bequest of Cecil John Rhodes, English politician and business magnate, for students from the Commonwealth, the United States, and Germany.
Rhodesia See: Zimbabwe.
Rhodesia and Nyasaland, Federation of, British federation in central Africa created in 1953 that included Southern Rhodesia, Northern Rhodesia, and Nyasaland.
Rhodesian ridgeback, also called African lion hound, hunting dog that originated in southern Africa.
Rhodium, chemical element, symbol Rh; for physical constants see Periodic Table.
Rhododendron, genus of mostly evergreen shrubs (family Ericaceae) found mainly in forests of the arctic and north temperate zones.
Rhombus, parallelogram in which the sides are of equal length but usually not at right angles to each other.
Rhubarb, name for plants (genus Rheum) of the buckwheat family.
Rib, in humans, one of the 24 long, flat, curved bones forming the wall of the chest.
Ribaut, or Ribault, Jean (1520?–65), French mariner who helped colonize Florida.
Ribbentrop, Joachim von (1893–1946), German Nazi leader, ambassador to the United Kingdom (1936–38) and foreign minister (1938–45).
Ribbon Falls, in the Yosemite Valley, one of the highest waterfalls in the world.
Ribbon worm, any of a group of elongated marine worms (phylum Nemertina), ranging in size from less than 1 in (2.5 cm) to 90 ft (27 m) long.
Ribera, Jusepe de (c.1590–1652), Spanish painter who lived after 1618 in Naples.
Ribicoff, Abraham A. (1910– ), U.S. public official, widely known as a champion of consumer protection.
Riboflavin See: Vitamin.
Ricardo, David (1772–1823), English economist, founder, with Adam Smith, of the classical school.
Ricci, Matteo (1552–1610), Italian Jesuit missionary.
Rice (Oryza sativa), grain-yielding annual plant of the grass family (Graminae).
Rice, Elmer (1892–1967), U.S. dramatist.
Rice, Grantland (1880–1954), U.S. journalist known as the first famous sportswriter.
Rice weevil See: Grain weevil.
Ricebird See: Bobolink.
Rich, Adrienne (1929– ), U.S. feminist poet whose primary themes are women's issues and sexuality and the problem of human communication.
Richard, name of three kings of England. Richard I (1157–99), called Coeur de Lion (the Lion Heart), was the third son of Henry II, whom he succeeded in 1189. He spent all but six months of his reign out of England, mainly on the Third Crusade. After taking Cyprus and Acre in 1191 and recapturing Jaffa in 1192, he was captured while returning to England and handed over to Holy Roman Emperor…
Richard the Lion-Hearted See: Richard.
Richard, Maurice (1921– ), Canadian-born hockey player.
Richards, Dickinson Woodruff (1895–1973), U.S. physiologist awarded, with A.F.
Richards, Ivor Armstrong (1893–1979), English literary critic.
Richardson, Elliot Lee (1920– ), U.S. lawyer and government official.
Richardson, Henry Hobson (1838–86), U.S. architect who pioneered an American Romanesque style.
Richardson, Samuel (1689–1761), English novelist, best known for his novels in epistolary form, especially Pamela; or, Virtue Rewarded (1740–41), the story of a servant girl's moral triumph over her lecherous master, and Clarissa Harlowe (1747–48), his tragic masterpiece, also on the theme of seduction.
Richelieu, Cardinal (Armand Jean du Plessis, Duc de Richelieu; 1585–1642), French cardinal, statesman, and chief minister to Louis XIII for 18 years.
Richelieu River, river in Quebec known for its scenic beauty.
Richfield (pop. 5,482), in central Utah, seat of Sevier County and site of various federal and state governmental agencies.
Richland (pop. 33,578), city in Washington and site of the U.S.
Richler, Mordecai (1931– ), Canadian writer.
Richmond (pop. 221,900), state capital of Virginia; capital of the Confederacy (1861–65).
Richmond (pop. 74,676), Calif., major West Coast port on the northeast shore of San Francisco Bay.
Richmond (pop. 41,349), city in Indiana and the seat of Wayne County, located on the Whitewater River.
Richmondtown, area in Staten Island, New York City, and county seat of Richmond County.
Richter, Conrad (1890–1968), U.S. writer of fiction and nonfiction known for his novels about life on the American frontier.
Richter, Hans (1843–1916), German conductor who presented the first performance of Wagner's Ring cycle at Bayreuth in 1876.
Richter, Johann Paul Friedrich (1763–1825), German humorous and sentimental novelist, who wrote as Jean Paul.
Richter scale, scale devised by C.F.
Richthofen, Manfred von (1892–1918), German aviator, nicknamed the Red Baron.
Rickenbacker, Eddie (Edward Vernon Rickenbacker; 1890–1973), U.S. air ace of World War I.
Rickets, deficiency disease of infancy due to lack of vitamin D, characterized by poor nutrition and changes in the bones (bowleggedness, knock-knees, etc.).
Rickettsia, name for organisms partway between bacteria and viruses.
Rickover, Hyman George (1900–86), Russian-born U.S. admiral who brought nuclear power to the U.S.
Ricksha See: Jinrikisha.
Ride, Sally Kristen (1951– ), U.S. astronaut and astrophysicist.
Rideau Canal, waterway in Ontario and Canadian historic site.
Ridgway, Matthew Bunker (1895–1985), U.S. military leader.
Ridley, Nicholas (c.1500–55), English Protestant martyr.
Riel, Louis (1844–85), Canadian rebel leader.
Riemann, Georg Friedrich Bernhard (1826–66), German mathematician, whose best-known contribution is the initiation of studies of non-Euclidean geometry.
Riemenschneider, Tilman (c. 1460–1531), German Gothic sculptor in wood and stone.
Rienzi, Cola di (1313–54), Italian popular leader.
Rifle, strictly, any firearm with a “rifled” bore—i.e., with shallow helical grooves cut inside the barrel.
Rift Valley See: Great Rift Valley.
Riga (pop. 875,000), capital of Latvia.
Rigel, one of the brightest stars in the galaxy.
Rigging See: Sailing.
Right of privacy See: Privacy, Right of.
Right of search, international law under which nations at war are allowed to search the vessels of neutral nations for contraband.
Right of way See: Easement.
Right whale See: Whale.
Right wing, conservative faction within a political group or party.
Right-to-work laws, laws enforced in 19 U.S. states requiring companies to maintain an “open shop” in which a person may not be prevented from working because he does not belong to a union.
Rights, Bill of See: Bill of rights.
Rights of Man, Declaration of the See: Declaration of the Rights of Man and the Citizen.
Riis, Jacob August (1849–1914), U.S. journalist and social reformer whose book How the Other Half Lives (1890) drew attention to slum conditions in New York City.
Riley, James Whitcomb (1849–1916), U.S. poet, known as the “Hoosier Poet.” The Old Swimmin' Hole and 'Leven More Poems (1883) was the first of his many popular collections of humorous and sentimental dialect poems.
Rilke, Rainer Maria (1875–1926), German lyric poet.
Rillieux, Norbert (1806–94), U.S. engineer who developed an improved method of manufacturing sugar (1846).
Rimbaud, Arthur (1854–91), French poet.
Rimsky-Korsakov, Nikolai (1844–1908), Russian composer.
Rinderpest, acute virus disease of cattle, common in North Africa and South Asia.
Rinehart, Mary Roberts (1876–1958), U.S. writer of popular detective stories, including The Circular Staircase (1908).
Ring, small circular band worn on the body as decoration.
Ringette, sport similar to ice hockey created for women.
Ringling brothers, five U.S. brothers who created the world's largest circus.
Ringtail, or cacomistle, member of the raccoon family found in North and Central America.
Ringworm, common fungus disease of the skin of humans and animals; it may also affect the hair and nails.
Rio de Janeiro (pop. 5,336,200), second largest city of Brazil, on the Atlantic coast about 200 mi (322 km) east of São Paulo.
Rio Grande, one of the longest rivers in North America, known in Mexico as the Rio Bravo del Norte.
Rio Madeira See: Madeira River.
Riot, unlawful rebellion against a public authority by a group of people, involving breach of the peace, destruction of property, and/or violence.
Rip Van Winkle, folk tale by U.S. author Washington Irving from his collection, The Sketch Book of Geoffrey Crayon, Gent. (1819–20).
Riparian rights, privileges accruing to owners of land on the edges of streams, rivers, and lakes.
Ripley, George (1802–80), U.S. social reformer and critic.
Rite of passage, ceremony within a community to mark an individual's achievement of a new stage in life (e.g., birth, puberty, marriage) and consequent change of role in the community.
Rittenhouse, David (1732–96), U.S. astronomer and mathematician who invented the diffraction grating, built two orreries, discovered the atmosphere of Venus (1768) independently of Lomonosov (1761), and built what was probably the first U.S. telescope.
Ritual See: Religion.
River, long channel of water. The ground beneath is called the bed; to either side are its banks. Rivers begin as headwaters overflowing from lakes or running down mountains as the snow melts, forming rills, brooks, and streams. The amount of river water depends on rainfall, since the river system provides the drainage for the surrounding land. The water runs downward to sea level, taking the shor…
River dolphin, any of four species of freshwater whales found in the waters of South America and Asia, belonging to the family Platanistidae.
River horse See: Hippopotamus.
Rivera, Diego (1886–1957), Mexican mural painter.
Rivers, Larry (1923– ), U.S. painter.
Riveting, joining of machine or structural parts, usually plates, by rivets.
Riviera, coastal region of the Mediterranean Sea in southeastern France and northwestern Italy.
Riyadh (pop. 1,380,000), Saudi Arabian city and seat of the Saudi royal family, about 240 mi (386 km) west of the Persian Gulf.
Rizal, José (1861–96), Philippine writer and patriot.
Rizzio, David (c. 1533–66), Italian musician, favorite of Mary, Queen of Scots.
RNA See: Nucleic acid.
Roach, fish belonging to the carp and minnow family, commonly found in the lakes and rivers of Europe.
Roach See: Cockroach.
Road, surfaced or unsurfaced path over which vehicles travel.
Roadrunner (Geococcyx californianus), large, slenderly built bird of the cuckoo family, found in arid regions in the southwestern United States and Mexico.
Roanoke (pop. 224,477), industrial, trade, transportation, and medical center, and one of the largest cities in Virginia.
Roanoke Island, island off the northeastern coast of North Carolina, 12 mi by 3 mi (19.3 km by 4.8 km) site of the first English settlement in North America (1585).
Roaring Twenties, period of the 1920s in the United States identified with restlessness and social reform.
Rob Roy (Robert MacGregor; 1671–1734), Scottish outlaw, romanticized in Sir Walter Scott's Rob Roy (1818).
Robbe-Grillet, Alain (1922– ), French novelist, originator of the French “new novel.” In works such as The Voyeur (1955), Jealousy (1957), and the screenplay for Last Year at Marienbad (1960), structure, objects, and events displace character and story.
Robber crab See: Hermit crab.
Robbins, Frederick Chapman (1916– ), U.S. virologist who shared the 1954 Nobel Prize in physiology or medicine with J.
Robbins, Jerome (1918–98), U.S. choreographer and director.
Robert's Rules of Order See: Parliamentary procedure.
Roberts, Kenneth Lewis (1885–1957), U.S. writer and Saturday Evening Post correspondent.
Roberts, Owen Josephus (1875–1955), associate justice of the U.S.
Roberts, Sir Charles George Douglas (1860–1943), Canadian writer.
Robertson, Oscar (1938– ), U.S. basketball player.
Roberval, Sieur de (Jean François de la Rocque; 1500?–60?), French explorer who led one of the first expeditions to colonize Canada.
Robeson, Paul (1898–1976), U.S. singer and stage and film actor.
Robespierre, Maximilien Marie Isidore (1758–94), fanatical idealist leader of the French Revolution.
Robin, vernacular name for various unrelated species of small birds with red breasts.
Robin Hood, legendary medieval English hero.
Robinson, Bill (1878–1949), popular U.S. dancer and entertainer, nicknamed “Bojangles.” He won national and international acclaim as a musical comedy performer and was featured in numerous Broadway shows and Hollywood films, including those in which he starred with Shirley Temple.
Robinson, Eddie (Edward Gay Robinson; 1919– ), the most victorious coach in college football history.
Robinson, Edwin Arlington (1869–1935), U.S. poet, known for his series of terse, sometimes bitter, verse characterizations of the inhabitants of the fictitious Tilbury Town.
Robinson, Frank (1935– ), U.S. baseball player and manager.
Robinson, Jackie (Jack Roosevelt Robinson; 1919–72), U.S. baseball player.
Robinson, James Harvey (1863–1936), U.S. historian.
Robinson, Joan Violet (1903–83), British economist, writer, and advocate of Keynesian economics.
Robinson, Sir Robert (1886–1975), English organic chemist awarded the 1947 Nobel Prize in chemistry for his pioneering studies of the molecular structures of alkaloids and other vegetable-derived substances.
Robinson, Sugar Ray (Walter Smith; 1921–89), U.S. boxer.
Robot, mechanical device equipped with sensing instruments for detecting input signals or environmental conditions, with a calculating mechanism for making decisions, and with a guidance mechanism for providing control.
Robusti, Jacopo See: Tintoretto.
Rocard, Michel Louis Leon (1930– ), French prime minister (1988–91).
Rochambeau, Comte de (Jean Baptiste Donatien de Vimeur; 1725–1807), French general who commanded French troops sent to help General George Washington in the American revolution.
Roche, Mazo De la See: De la Roche, Mazo.
Rochester (pop. 245,000), large industrial city in upstate New York, on the banks of the Genesee River, near its confluence with Lake Ontario.
Rock, hard, solid matter of the earth's crust, sometimes a combination of one or more minerals. Rock may occur close to the earth's surface or deep underground. Rocks are classified according to their origin. Igneous rock forms when magma (molten material deep within the earth) rises toward the surface and cools. Intrusive igneous rock, such as granite, results if the magma solidifie…
Rock festival See: Rock music.
Rock music, the dominant popular music since the late 1950s. Rock music first emerged in the mid-1950s as rock 'n' roll, a hybrid evolving from a sophisticated blues style called rhythm and blues, which often used amplified instruments to produce a heavy beat. The first national rock 'n' roll hit—and the one that probably gave the genre its name—was ȁ…
Rock oil See: Petroleum.
Rockefeller, family of U.S. financiers and politicians. John Davison Rockefeller (1839–1937) entered the infant oil industry in Cleveland, Ohio, at the age of 24 and ruthlessly unified the oil industry into the Standard Oil Trust. He devoted a large part of his later life to philanthropy, creating the Rockefeller Foundation. John Davison Rockefeller, Jr. (1874–1960), only son of John…
Rockefeller Foundation, U.S. philanthropic foundation.
Rocket, form of jet-propulsion engine in which the substances (fuel and oxidizer) needed to produce the propellant gas jet are carried internally. Working by reaction, and being independent of atmospheric oxygen, rockets are used to power interplanetary space vehicles. In addition to their chief use to power missiles, rockets are also used for supersonic and assisted-takeoff airplane propulsion, a…
Rocket, The, first locomotive powered by steam.
Rocket, model, or space model, small-scale working replica of the kind of rocket used in military and space programs.
Rockford (pop. 283,719), second-largest city in Illinois.
Rockne, Knute Kenneth (1888–1931), U.S. football coach.
Rockwell, Norman (1894–1978), U.S. illustrator, known for his realistic and humorous scenes of U.S. small town life.
Rocky Mountain goat (Oreamnos americanus), goatlike herbivorous mammal closely related to the antelope.
Rocky Mountain National Park, natural wild area in north central Colorado, in the heart of the Rocky Mountains.
Rocky Mountain spotted fever, acute febrile disease caused by Rickettsia rickettsii and transmitted by ixodid ticks.
Rocky Mountains, principal range of the western region of North America.
Rococo, 18th-century European artistic and architectural style.
Rodent, largest order of mammals, including some 1,500 species of mice, rats, porcupines, and squirrels.
Rodeo, in the United States and Canada, contest and entertainment based on ranching techniques; it derives from late-19th-century cowboy meets held to celebrate the end of a cattle drive.
Rodgers, Richard (1902–79), U.S. songwriter and composer.
Rodin, Auguste (1840–1917), French sculptor.
Rodney, Caesar (1728–84), American statesman.
Rodrigo Díaz See: Cid, El.
Roebling, John Augustus (1806–69), U.S. engineer who pioneered modern suspension bridge design.
Roemer, Olaus (1644–1710), Danish astronomer, the first to show that light has a finite velocity.
Roentgen, Wilhelm Conrad (1845–1923), German physicist, recipient (1901) of the first Nobel Prize in physics for his discovery of X rays.
Roethke, Theodore (1908–63), U.S. poet who won a Pulitzer Prize for The Waking (1953) and a National Book Award for Words for the Wind (1958).
Rogers, Carl Ransom (1902–87), U.S. psychotherapist, who instituted the idea of the patient determining the extent and nature of his course of therapy, the therapist following the patient's lead.
Rogers, John (1829–1904), U.S. sculptor known for realistic figural groups, such as The Slave Auction.
Rogers' Rangers, U.S. commando unit that fought on the side of the British during the French and Indian War.
Rogers, Will (William Penn Adair; 1879–1935), U.S. humorist known for his homespun philosophy and mockery of politics and other subjects previously considered “untouchable.” Part Irish and part Cherokee, he became famous in the Ziegfeld Follies of 1916.
Roget, Peter Mark (1779–1869), English scholar and physician, author of the definitive Thesaurus of English Words and Phrases (1852).
Roh Tae Woo (1932– ), South Korean president (1988–92), succeeded by Kim Young Sam.
Rohrer, Heinrich (1933– ), Swiss physicist, winner with Gerd Binnig and Ernst Ruska of West Germany of the Nobel Prize for physics (1986) for his contribution to the development of the scanning tunneling microscope (STM) that allows scientists to view individual atoms.
Roland, one of Charlemagne's commanders, hero of the Song of Roland.
Roland de la Platière, husband and wife French revolutionaries.
Rolfe, John (1585–1622), early English settler in Virginia who married the Indian princess Pocahontas (1614).
Rolland, Romain (1866–1944), French writer, who won the 1915 Nobel Prize for literature.
Roller, species of bird belonging to the roller family.
Roller skating, popular source of sport and recreation.
Rolling Stones, influential rock band from Britain.
Romains, Jules (Louis Farigoule; 1885–1972), French author and exponent of unanimism, or the collective personality.
Roman Catholic Church, major branch of the Christian church consisting of Christians in communion with the pope. It comprises the ecclesiastical organization that remained under papal obedience at the Reformation, consisting of a hierarchy of bishops and priests, with other officers such as cardinals. Roman Catholicism stresses the authority of tradition and the church (through ecumenical councils…
Roman Circus See: Rome, Ancient.
Roman Empire See: Rome, Ancient.
Romance, literary term identified with fiction usually depicting idealized love.
Romance languages, one of the main groups of the Indo-European languages.