In the Kyrie and Agnus Dei, the single cursus of the cantus firmus is split right down the middle and
alternates with a subsection in which “the tenor is silent” (tenor tacet) to quote the rubric in the
choirbook from which such tenorless middle sections as the Christe eleison or the Agnus Dei II get their
generic name. That alternation supplies the requisite “A–B–A-ness” to delineate the textual form. The
sense of climax is achieved in every movement past the Kyrie by accompanying the cantus firmus, on its
resumption, with voices notated in diminution. Speeding along against an unchanged tactus in the tenor,
they reach a really dizzy pitch of virtuosity.
Following a custom already observed in the Caput Masses, Busnoys provides a cap to the entire
Ordinary setting in the concluding Agnus Dei by manipulating the cantus firmus with a special “canon” or
transformation rule. (“Gimmick” might actually be the best translation, flippant though it may seem.) The
tenor appears to carry the tune in its usual form, but a jesting puzzle-rubric—Ubi thesis assint ceptra, ibi
arsis et e contra (“Where [the armed man’s] scepter is raised, there go lower and vice versa”)—directs
the singers to exchange roles with the basses, who sing the cantus firmus not only down an octave but also
with all the intervals inverted. Ex. 12-12 shows the end of the Mass.
EX. 12-12 Antoine Busnoys, Missa L’Homme Armé, last Agnus Dei, mm. 14–22