A song is a single (and often standalone) work of music intended to be sung by the human voice with distinct and fixed pitches and patterns using sound and silence and a variety of forms that often include the repetition 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 are often referred to as folk songs. Songs that are composed for professional singers 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 performances. Songs are performed live and recorded. Songs may also appear in plays, musical theatre, stage shows of any form, and within operas.