Here are entered collections of compositions for various combinations of two instruments. Collections of compositions for the same two specified instruments, and individual works, are entered under Bassoon and piano music, Violin music (Violins (2)), and similar headings. Collections of compositions and individual works for two unspecified instruments, or for a particular specified instrument and an unspecified instrument, are entered under Duets followed by a parenthetical qualifier indicating the medium of performance.Compositions for two solo voices are entered under Sacred vocal duets or Vocal duets. --headings of the type Piano music (4 hands) and similar headings for two performers on other keyboard instruments