The effect of anguished mortality is heightened in the Mass through additional chromaticism: a
specifically signed F-sharp right before a specifically signed B-flat in each of the two voices in imitation.
The resulting diminished fourth was something that might have been introduced into the motet by daring
singers exercising their rights, so to speak, when it came to musica ficta—especially if those singers
knew Du Fay’s very late chanson Hélas mon deuil, a ce coup sui je mort (“Alas, my woe, at this blow I
die”), where the same tortured interval is demanded for the very same expressive purpose (Ex. 13-5). No
contrapuntal situation could demand such a rash of accidentals. It must answer to someone’s specific
expressive intent.
EX. 13-5 Guillaume Du Fay, Hélas mon deuil, mm. 1–8