Partbooks

The manuscript or printed books of the 15th and 16th centuries, each containing the music for an individual voice of a polyphonic composition.  The usual number of partbooks in a set is four: Cantus (Discantus, Superius); Altus; Tenor;  Bassus (Basis).  Publication in partbooks clearly indicates that the music thus written or published is ensemble music, either vocal, or instrumental.