ACCOMPANIMENT (i.e. that which " accompanies ")  , 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 . The accompaniment may be obbligato or ad libitum, according as it forms an essential part of the composition or not . The term obbligato or obbligato accompaniment is also used for an independent instrumental solo accompanying a vocal piece . 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 the older masters; such are Mozart's " additional " accompaniments to Handel's Messiah or those to many of the elder Bach's works by Robert Franz . In common parlance any support given, e.g. by the piano, to a voice or instrument is loosely called an accompaniment, which may be merely " vamped " by the introduction of a few chords, or may rise to the dignity of an artistic composition . In the history of song the evolution of the art side of an accompaniment is important, and in the higher forms the vocal and instrumental parts practically constitute a duet, in which the instrumental part may be at least as important as that of the voice .