A song is a musical composition intended to be vocally performed by the human voice. This is often done at distinct and fixed pitches (melodies) using patterns of sound and silence. Songs contain various forms, such as those including the repetition and variation of sections. Written words created specifically for music or for which music is specifically created, are called lyrics. If a pre-existing poem is set to composed music in classical music it is an art song. Songs that are sung on repeated pitches without distinct contours and patterns that rise and fall are called chants. Songs in a simple style that are learned informally "by ear" are often referred to as folk songs. Songs that are composed for professional singers who sell their recordings or live shows to the mass market are called popular songs. These songs, which have broad appeal, are often composed by professional songwriters, composers and lyricists. Art songs are composed by trained classical composers for concert or recital performances. Songs are performed live and recorded on audio or video (or, in some cases, a song may be performed live and simultaneously recorded). Songs may also appear in plays, musical theatre, stage shows of any form, and within operas, films, and TV shows. A song may be for a solo singer, a lead singer supported by background singers, a duet, trio, or larger ensemble involving more voices singing in harmony, although the term is generally not used for large classical music vocal forms including opera and oratorio, which use terms such as aria and recitative instead. A song can be sung without accompaniment by instrumentalists (a cappella) or accompanied by instruments. In popular music, a singer may perform with an acoustic guitarist, pianist, organist, accordionist, or a backing band. In jazz, a singer may perform with a single pianist, a small combo (such as a trio or quartet), or with a big band. A Classical singer may perform with a single pianist, a small ensemble, or an orchestra. In jazz and blues, singers often learn songs "by ear" and they may improvise some melody lines. In Classical music, melodies are written by composers in sheet music format, so singers learn to read music. Songs with more than one voice to a part singing in polyphony or harmony are considered choral works. Songs can be broadly divided into many different forms and types, depending on the criteria used. Through semantic widening, a broader sense of the word "song" may refer to instrumentals, such as 19th century "Song Without Words" pieces for solo piano.