Aaron Copland reached his special status in American classical music by mastering the gift to be simple. Or at least to appear simple -- to generate a spontaneous, uncomplicated response in his audiences. He will be remembered -- not only in New York and Omaha, but in London, Berlin and Tokyo -- as the composer who, more than any other, gave American music to the world in classical forms.

He may or may not have been the best American classical composer of the mid-20th century; that idea is subject to definition and endless debate. And he may not have influenced foreign composers as much as Elliott Carter, Philip Glass and John Cage. But a nearly universal consensus has labeled Copland, who died yesterday at age 90, the most thoroughly American composer of his time.

Virgil Thomson (a strong runner-up in the all-American composer sweepstakes) learned old hymn tunes by playing them on his church organ back home in Kansas City, where, occasionally, he may even have heard a cowboy singing some of the cowboy songs he later used in his soundtrack for "The Plow That Broke the Plains."

In contrast, Copland's detractors have called him a "cowboy from Brooklyn." This may sound rather harsh and flippant, but Copland was born in Brooklyn and undoubtedly got his cowboy tunes and other folklore, such as his two sets of "Old American Songs," from the printed page. More than where it came from, though, Copland's music is important for where it has gone.

His influence can be seen not only in "Old American Songs" but also, earlier and more spectacularly, in his ballet scores, produced at a time when American dance was coming of age: the square-dance lilt and rhythm in the "Hoe-Down" from "Rodeo" (1942); the variations on "I Ride an Old Paint" from "Billy the Kid" (1938); and, above all, the Shaker hymn tune "Simple Gifts," which climaxes "Appalachian Spring" (1943-44). He also used an old ballad tune for a set of orchestral variations in "John Henry" (1940), which deserves to be better known, and the 19th-century tunes "Camptown Races" and "Springfield Mountain" in "Lincoln Portrait" (1942).

"Simple Gifts," also included in his "Old American Songs," can be taken as a major theme of Copland's career -- not only the tune that he used so ingeniously, but the words that might have been his motto: " 'Tis a gift to be simple, 'tis a gift to be free,/ 'Tis a gift to come down where you ought to be."

In his book "The New Music" (1968), Copland could have been talking about himself in a passage in which he discussed the musical philosophy of Thomson. "While most composers on the musical left are busily engaged in inventing all sorts of new rhythmic and harmonic devices, intent on being as original and different as possible, Thomson goes to an opposite extreme and deliberately writes music as ordinary as possible ... from the conviction that modern music has forgotten its audience almost completely, that the purpose of music is not to impress and overwhelm the listener but to entertain and charm him. Thomson seems determined to win adherents to music through music of an absolute simplicity and directness."

Thomson, in turn, has described Copland's orchestration in (quite accurate) terms that could also apply to his own work: "plain, clean-colored, deeply imaginative ... theatrically functional." A Copland comment on this subject is even more succinct: "Orchestral know-how consists in keeping instruments out of each other's way." His use of words was like his use of instruments -- he chose simplicity and straightforwardness where others chose jargon; he understated his cleverness while others tried to proclaim theirs.

William Schuman, a fellow composer almost as honored as Copland, once asked him whether he ever got tired of the honors that were heaped upon him, including the Pulitzer Prize in 1944 and the New York Critics Circle award in 1945, the Academy Award in 1950, the Gold Medal of the American Academy of Arts and Letters in 1956, the Presidential Medal of Freedom in 1964, honorary degrees and other citations from Yale, Harvard, Princeton, Brandeis, Oberlin and other colleges; medals and other awards from West Germany, England, Italy, Argentina, Chile and other countries and more than 50 television appearances. "You underestimate my capacity," said Copland. But his closest acquaintances agree that he was essentially a modest man.

He was once the center of a controversy, when his "Lincoln Portrait" was proposed as a musical selection for Dwight Eisenhower's inauguration in 1953, and a member of Congress, Rep. Fred Busby of Illinois, called the National Symphony and said, "You cannot play the 'Lincoln Portrait' by Aaron Copland because he is a Communist sympathizer." There is no question that Copland was associated briefly with some politically progressive organizations in the 1930s, but there is even less question that the "Lincoln Portrait" (for which he prepared the words, mostly drawn from Lincoln's writings, as well as the music) is as solidly, heart-on-sleeve American in spirit as any work of art ever produced on this continent.

This was at the height of the McCarthy era, and the "Lincoln Portrait" was withdrawn from the inaugural program. Later, Busby was asked whether American military bands should stop playing popular excerpts from "Rodeo," "Billy the Kid" and other Copland works -- music that was often on their programs and always well received. "Indeed," he said, "that is something we would have to consider."

In an interview nearly 30 years later, Copland was philosophical about this episode. "You just have to wait for history to take its course," he said. "Things like that die out very gradually ... if you live long enough -- and I certainly can't complain in that respect -- and if you are persistent. You know, one has to be persistent to get anywhere in this life." He said he had no bitterness because it was "so long ago" and "I took it as one of the facts of life."

Curiously, Copland learned how to become the standard-bearer of American music in Paris, where he studied with Nadia Boulanger (1921-24), and in Mexico City, where he went in 1932 to work with Carlos Chavez, Mexico's leading composer and a notable arranger of Mexican folk music. Before he began using the folk material of his own country, Copland first worked with Mexican melodies in "El Salon Mexico" (1933-36). Besides developing the style and thematic material that made him famous, Copland experimented with other idioms that ranged from jazz (in his 1927 Piano Concerto) to atonality (in the 1930 Piano Variations and, later and more systematically, the 1950 Piano Quartet).

While his simple, homespun Americana reached an audience of millions, Copland also composed music of deep complexity and high quality in the most uncompromisingly modern idioms. Probably always destined to appeal only to a small audience of connoisseurs, this other side of Copland's output includes three major piano works, well spaced through his career: the Variations (1957), he Sonata (1941) and the Fantasy (1957). There is also distinguished chamber music: the Sextet (1937), the Violin Sonata (1943) and the Nonet (1961), as well as orchestral music with substance to match such rarefied titles as "Connotations" (1962) and "Inscape" (1967). Vocally, his "12 Poems of Emily Dickinson" are a cornerstone of the American art-song repertoire, but are not heard nearly as often as the more popular (and less original) "Old American Songs." It would be incorrect to call this material the "unknown" Copland; all of it has been recorded in good performances, and it is just possible that a century from now these less familiar works will keep his name alive. Asked which of his works he expects will survive, he once said, "You just can't worry about that."

Actually, the simplicity in much of his more popular work is more apparent than real. There is a fine sophistication in the free but coherent structures and subtly shifting instrumental colors of "Appalachian Spring," but it is an art that conceals art; it works on the audience's imagination without calling attention to itself. It sounds utterly plain, like the lives of the Pennsylvania farmers who are the subject of the ballet. But, like much of Copland's best theatrical music, it is as subtly inflected as the voice of a fine actor reciting Shakespeare.

In his fifties, Copland began to compose less and devote more of his time to conducting -- after noticing that some conductors were earning more for playing his music than he had earned for composing it, he once said. He began with his own music, later branched out into the music of other American composers and although he never became a baton virtuoso, he eventually acquired a fair level of ability.

"I couldn't really start conducting until after Serge Koussevitzky {conductor of the Boston Symphony and one of Copland's major supporters} died in the early '50s," he said. "He just wouldn't have permitted it. You know, at times he almost ran my life. When I mentioned conducting, he ordered me back home and said, 'You keep writing. You should stay home and compose. Don't waste time conducting.' I think he was right. But conducting is fun. Obviously, I don't stand up there and conduct with the authority of the greatest conductors, but I certainly know my own music well."

He conducted the National Symphony Orchestra, always with enthusiastic audiences, about once a year in the 1970s and early '80s, sometimes in a birthday concert, sometimes outdoors on the Capitol grounds, until failing health forced him to retire. He celebrated his 80th birthday, Nov. 14, 1980, with the National Symphony at the Kennedy Center; he conducted "Appalachian Spring" and his Piano Concerto on that occasion, and read the text of the "Lincoln Portrait" while Leonard Bernstein conducted.

Copland's quiet, unassuming skill as a composer suited him ideally for the humble but demanding craft of writing movie soundtrack music. He set new standards for Hollywood with his eight works in this genre.

His special feeling for movies was recalled by Bernstein in 1979, when Copland received a Kennedy Center Honor, along with Henry Fonda, Tennessee Williams, Ella Fitzgerald and Martha Graham. In the 42 years he had known Copland, Bernstein said, he had seen him weep only once and get angry only once -- both on the same evening. They were watching a Bette Davis movie together, when Bernstein made a derogatory remark about the film. Copland responded, "Can't you shut up!" and, when Bernstein looked over at his friend, he saw that he was weeping.

Throughout his long career, Copland was a tireless promoter of the music of other American composers through concerts, festivals, publication of scores, articles in Modern Music magazine and such books as "What to Listen for in Music" (1939), "Our New Music" (1941, revised as "The New Music," 1968), and "Music and Imagination" (1952). "Music and Imagination" was based on the 1951-52 Norton Lectures delived at Harvard University while he was in residence there. He also taught at Harvard in 1933 and 1944, and at the Boston Symphony's summer school at Tanglewood (in Lenox, Mass.) every year from its foundation in 1940 until his retirement in 1965, serving as the chairman of the faculty from 1957 on. He was a mentor for younger composers as diverse as Bernstein, Ned Rorem, David Diamond, David del Tredici and Toru Takemitsu. Following the lead of his own teacher, Boulanger, he encouraged each of these disciples to develop in his own way, not to become a Copland copy.

This visionary artist and solid craftsman, born when the century was new, stamped his image firmly on its music. Beneath the simple exterior of that image lie complexities that his mass audience has hardly begun to explore. If there was one tragedy in his life, it was probably the fact that he did not manage to bring that audience along into the more rarefied regions of his art. But the fact that he found that audience and took it as far as he did makes him unique among the composers of his time.