In music, accompaniment is the art of playing along with an instrumental or vocal soloist or ensemble, often known as the lead, in a supporting manner. The accompaniment can be performed by a single performer—a pianist, organist, or guitarist—or it can be played by an entire ensemble, such as a symphony orchestra or string quartet (in the Classical genre) or a backing band or rhythm section (in popular music) or a Big Band or organ trio (in jazz). It may be considered the background to the foreground melody. The term "accompaniment" is also used to describe the composed music, arrangement, or improvised performance that is played to back up the soloist. In most Classical styles, the accompaniment part is written by the composer and provided to the performers in the form of sheet music. In jazz and popular music, the backing band or rhythm section may improvise the accompaniment based on standard forms—in the case of a small blues band or jazz band playing a 12 bar blues progression--or the band may play from a written arrangement in a jazz Big Band or in a musical theater show.