Cuba had been on Copland's mind for some time. A commission to write a piece for the Leage of Composers' 20th anniversary gave him the incentive to compose ''Danzón Cubano'' for two pianos. It is based on Cuban dance rhythms, particularly the danón, a stately dance, somewhat like the waltz. In two contrasting sections, Copland makes use of four Cuban dances with syncopated beats and polyrhythms. Copland explained, ''I did not attempt to reproduce an authentic Cuban sound, but felt free to add my own touches of displaced accents and unexpected silent beats.''