'Tis the gift to be simple, 'tis the gift to be free.

'Tis the gift to come down where we ought to be.

AARON Copland did not write those words or the simple, memorable tune that accompanies them, but they are indelibly linked to his name. In a way, they can be read as a manifesto--an explanation of why Copland's music is more popular than any other American classical composer's. He found the gift to be simple, and his reward is a status in our concert life comparable to that of Tchaikovsky's.

The music for "Simple Gifts," as arranged by Copland in a set of variations for his ballet "Appalachian Spring," will be heard this weekend in Washington, where it had its first performance in 1944--and it will be heard in the 1944 version for 13 instruments, long overshadowed by Copland's later arrangement for large orchestra. The Twentieth Century Consort will play it tomorrow evening at the Hirshhorn Gallery in the final concert of its regular season. Sunday in the Library of Congress, where "Appalachian Spring" had its premiere, the program will be repeated in an invitation-only concert to be videotaped for later broadcast on Channel 26.

Even with "Appalachian Spring" (in a relatively unfamiliar form), the program could be titled "The Unknown Copland." It will include his Sextet for Clarinet, Piano and Strings (based on his Short Symphony), the spiky, modernistic Piano Variations, dating from 1930, and the simple, lyrical Duo for Flute and Piano that he composed in 1971 in memory of flutist William Kincaid--music spanning more than four decades and a variety of styles. In a sense, it will be a tribute to one of the neglected composers of our time--Copland the chamber musician.

Copland is as old as the 20th century (he was born in 1900), and he has been active as a composer for about half of the century, but he is known to the general public almost entirely through a handful of orchestral works composed in a six-year period: "Billy the Kid" (1938), "Rodeo" (1942), "A Lincoln Portrait" (1942), "Fanfare for the Common Man" (1944) and "Appalachian Spring" (1944). All are notable for the unique instrumental flavor Copland puts into his orchestral music, and all but the "Fanfare" use American folk melodies or idioms. By concentrating on this small, specialized corner of his work, American music-lovers are depriving themselves of a variety of musical pleasures and Copland of the well-balanced public image he deserves.

Copland is not a prolific composer--at least not like Alan Hovhaness, who has produced more than 40 symphonies--but he has to his credit a half dozen chamber works, three major piano compositions, a couple of song cycles and a wealth of orchestral works that should be heard more often.

He went through a jazz-influenced period in the 1920s, memorably enshrined in his Piano Concerto and his "Music for the Theatre." He explored eclectic forms of modernism in the early 1930s--notably in his Piano Variations and his "Statements for Orchestra." Before settling on American folk music and hitting the jackpot, he tried his hand at other folk idioms in "El Salon Mexico," the "Danzon Cubano" and the "Vitebsk" Trio, his only explicit tribute to his Jewish heritage and a work in which he effectively uses quarter tones.

After his folk music splurge, Copland returned to abstract forms and higher levels of complexity with his Third Symphony (1946), his Clarinet Concerto (1948) and the Piano Fantasy (1957). A flirtation with the 12-tone system began with the intriguing Piano Quartet of 1950 and influenced several significant, seldom-heard pieces for orchestra: "Connotations" (1962), "Music for a Great City" (1964) and "Inscape" (1967).

In all the styles he has explored, Copland shows complete control of his material and imprints it with the unmistakable mark of his personality. Compared with other living American composers (Ellen Taafe Zwilich who won this year's Pulitzer, for example), perhaps he is not really neglected. All of his major works have been recorded, and most of them are performed once in a while. But it is quite likely that his best work is getting the least attention--that 100 years from now, Copland will be remembered more for the Piano Variations, the Third Symphony and the "Connotations" than for "Rodeo" or "Billy the Kid." On that question, we will have to wait and see. Meanwhile, the Twentieth Century Consort will give a timely reminder this weekend that there is a lot more to Copland than usually meets the ear.