The Benedictus section of the Sanctus, which seems to have customarily followed the Elevation of the Host when the Ordinary was sung in polyphony, is traditionally cast as a duet without cantus firmus, and Obrecht’s setting conforms to this convention. Somewhat unusual, however, is the division into two distinct subsections, the first centered on C, the second on G, and both commencing with the same four-note motive presented in imitation.
For more information, see:
Crocker, Richard L., and David Hiley. 2001 “Sanctus.” Grove Music Online. 30 Jul. 2018. www.oxfordmusiconline.com.
Hiley, David. Western Plainchant: A Handbook. Oxford: Oxford University Press, 1993. See in particular pp. 161-65.
Strohm, Reinhard. Music in Late Medieval Bruges. New York: Oxford University Press. 1985. See in particular pp. 146-48.
Wegman, Rob C. Born for the Muses: The Life and Masses of Jacob Obrecht. New York: Oxford University Press, 1994. See in particular pp. 169-74.