A Dictionary of Music and Musicians/Sextus

SEXTUS (Pars sexta, Sextuplum; Eng. The Sixth Voice, or Part). In the Part-Books of the 15th and 16th centuries, four Voices only were, as a general rule, mentioned by name; the Cantus, Altus, Tenore, and Bassus. When a fifth Voice was needed, it was called Quintus, or Pars Quinta, and corresponded exactly, in compass, with one of the first four. When yet another Voice was added, it was called Sextus, or Pars Sexta; and corresponded in compass with another original Voice-Part. The extra Part, therefore, represented sometimes an additional Treble, sometimes an Alto, sometimes a Tenor, and sometimes a Bass; and always corresponded, in compass, with some other Part of equal importance with itself.

[ W. S. R. ]