An art song is a vocal music composition, usually written for one voice with piano accompaniment, and usually in the classical art music tradition. By extension, the term "art song" is used to refer to the collective genre of such songs (e.g., the "art song repertoire"). An art song is most often a musical setting of an independent poem or text, "intended for the concert repertory" "as part of a recital or other relatively formal social occasion". While many pieces of vocal music are easily recognized as art songs, others are more difficult to categorize. For example, a wordless vocalise written by a classical composer is sometimes considered an art song and sometimes not.