Fig 1. Singer and piano accompanist
An accompaniment is a vocal or instrumental part that provides rhythmic and/or harmonic support, or background, for a lead melodic line. Many different styles and types of accompaniment are found in different genres and styles of music. In homophonic music, which is the mostly used approach to accompaniment in popular music, a vocal melody is supported by subordinate chords. In popular music and traditional music, the accompaniment parts typically provide the beat for the music and outline the chord progression of the song or instrumental piece.
The accompaniment for a vocal melody or instrumental solo can be played by a single musician playing an instrument such as piano or guitar. While any instrument can in theory be used as an accompaniment instrument, keyboard and guitar-family instruments tend to be used if there is only a single instrument, as they can play chords and basslines simultaneously. (Chords and a bassline are easier to play simultaneously on keyboard instruments, but a fingerpicking guitarist can play chords and a bassline simultaneously on guitar). A solo singer can accompany themself by playing guitar or piano while they sing.
Alternatively, the accompaniment to a vocal melody or instrumental solo can be provided by a musical ensemble, ranging in size from a duo (e.g., cello and piano; guitar and double bass); a trio (e.g., a power trio of electric guitar, electric bass, and drum kit); a quartet (e.g., a string quartet in Classical music can accompany a solo singer; a rock band or rhythm section in rock and pop; a jazz quartet in jazz); all the way to larger ensembles, such as concert bands, Big Bands (in jazz), pit orchestra in musical theater; and orchestras, which, in addition to playing symphonies, can also provide accompaniment to a concerto solo instrumentalist or to solo singers in opera. With choral music, the accompaniment to a vocal solo can be provided by other singers in the choir, who sing harmony parts or countermelodies.