(Ger. Bassett, Bassetgen; It. bassetto).
Diminutive of Bass (i). The lowest part of a passage or composition lacking a bass part and which executes the musical bass in higher register. It was defined by Praetorius in the Syntagma musicum, iii (1618), 121–2:
the lowest voice in a high-register chorus, which executes the fundament and whose structure resembles a true bass; … any lowest-sounding part in a high register … in both concertos and motets [i.e. in both stile moderno and stile antico], whether soprano, alto, or tenor, as we see most often in fugues.
This usage, peculiar to the Baroque era, frequently appeared in discussions of basso continuo, where it denoted a passage notated in a C clef or G clef rather than the usual F clef. ‘Bassetto’ also denoted the lowest part of each separate chorus in a polychoral composition.
F.T. Arnold: The Art of Accompaniment from a Thorough-bass (London, 1931/R), 373ff
JAMES WEBSTER