usual form in which vocal or instrumental polyphonic music was handwritten or printed in the 15th and 16th centuries. Each partbook contained the notation of only one voice, or part. The parts of madrigals, however, were sometimes published crosswise on single sheets, which allowed each of the singers seated around a rectangular table to sing from his particular part. Most commonly there were four partbooks: cantus (also discantus or superius), altus, tenor, and bassus; additional parts were either indicated quinta vox, etc., or were subdivisions of one of the principal parts—e.g., cantus I and cantus II. The practice of having musicians perform from their individual parts has continued in chamber and orchestral music.