A song is a relatively short musical composition for the human voice (commonly accompanied by other musical instruments), which features words (lyrics). It is typically for a solo singer, though may also be a duet, trio, or for more voices (works with more than one voice to a part, however, are considered choral). The words of songs are typically of a poetic, rhyming nature, although they may be religious verses or free prose. Songs can be broadly divided into many different forms, depending on the criteria used. One division is between "art songs", "popular music songs", and "folk songs". Other common methods of classification are by purpose (sacred vs secular), by style (dance, ballad, Lieder, etc.) or by time of origin (Renaissance, Contemporary, etc).