A song, most broadly, is a single (and often standalone) work of music that is typically 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. The word "song" is widely used by people in the popular music industry to describe any musical composition, whether sung or played only by instruments. "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 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. 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. 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, depending on the criteria used. Songs may be written for one or more singers to sing without instrumental accompaniment or they may be written for performance with instrumental accompaniment. The accompaniment used for a song depends on the genre of music and, in classical styles, the instructions of the composer as set out in the musical score. Songs may be accompanied by a single accompanist playing piano or guitar, by a small ensemble (e.g., a jazz quartet, a basso continuo group (in the case of Baroque music), a rock or pop band or a rhythm section) or even a big band (for a jazz song) or orchestra (for a classical aria). One division is between "art songs", "pop songs" and traditional music which includes "folk songs" and early blues songs. Other common methods of classification are by purpose (sacred vs secular), by style (dance, ballad, Lied, etc.), or by time of origin (Renaissance, Contemporary, etc.). Songs may be learned and passed on "by ear" (as in traditional folk songs); from a recording or lead sheet (in jazz and pop) or from detailed music notation (in classical music). Some instrumental music which is played in a singing style may be named a song, e.g., Mendelssohn's Songs Without Words for solo piano.