A song is a musical form of expression based on sound and silence, generally considered a single (and often standalone) work of music intended to be sung by the human voice with distinct and fixed pitches, patterns, and form. Written words created specifically for music or for which music is specifically created, are called lyrics. If poetry, a pre-existing poem, is set to composed music in classical music, that is an art song. Songs that are sung on repeated pitches without distinct contours and patterns that rise and fall are called chants. Chants may be slightly or highly ornamented. Songs may be sung by one singer or more than one, by a singer with background singers who accompany with minor parts, or by a group. Songs composed for personal use, for casual group activities, in simple style, are referred to as folk songs. Songs that are composed for professional entertainers are called popular songs, in that they do not require an education to necessarily appreciate, and that they have broad appeal to many people. These songs are composed with the intent to earn money by professional songwriters, composers and lyricists. Art songs are composed by trained classical composers for concert performance. Songs may also appear in plays, musical theatre, stage shows of any form, and within operas.