A song is a musical composition intended to be performed by the human voice. This is often done at distinct and fixed pitches 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 composed 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.