Duo was commissioned by seventy pupils and friends of the celebrated flutist William Kincaid after his death in 1967. Copland described it as lyrical and in a pastoral style. "Lyricism seems to be built into the flute," he wrote. Duo is in three movements. "The whole is a work of comparatively simple harmonic and melodic outline, direct in expression. Being aware that many of the flutists who were responsible for commissioning the piece would want to play it, I tried to make it grateful for the performer...it requires a good player." The piece has become a standard in the repertoire of flutists worldwide and is also available in a version for violin and piano.