A choir (sometimes 'chorus') is a ensemble of singers who perform together, usually led by a conductor. They can be classified by the types and number of voices that make them up, with the most usual type being the mixed SATB choir. It is not unusual to find all-female or all-male choirs (in all-male choirs sometimes boys and countertenors may be used to performer SATB pieces). Commonly, choirs are associated to either a church, theater, school, or community. They can perform with or without instrumental accompaniment.