ACCOMPANIMENT, a musical term for that part of a vocal or instrumental composition added to support and heighten the principal vocal or instrumental part ; either by means of other vocal parts, single instruments or the orchestra. Owing to the early custom of only writing the accompaniment in outline, by means of a "figured bass," to be filled in by the performer, and to the changes in the number, quality and types of the instruments of the orchestra, "additional" accompaniments have been written for the works of some of the older masters ; such are Mozart's "addi tional" accompaniments to Handel's Messiah (see article on HANDEL) . In the case of songs an accompaniment may consist merely of a few chords of the simplest kind or take the most elab orate form and constitute the most striking feature of the com position. In the latter case the performance of such accompani ments calls for the highest skill and constitutes a distinctive branch of the pianist's art, demanding the amplest powers of exe cution and first-class musicianship, especially when the player is called upon, as sometimes happens, to transpose a difficult ac companiment at sight for the convenience of the singer. Accom panying is also an important branch of conducting, in the case of concertos, choral music, and so forth.