The soprano and bass, which have sat out this duet, now re-enter the texture with a stretto of their own, one whose ending overlaps with the beginning of the piece’s second rising cascade. (This one doesn’t mount continuously from the bass, as the first did, but in a pair of upward moves, from alto to soprano and bass to tenor.)

Like its predecessor, it generates a chain of cadences:

Bach now wends his way, via a more freely contrapuntal stretch, to this:

another cascade, of course, but of a higher artistic order altogether. Its stature is suggested by the number of things there are to say about it. The most basic is that it doesn’t rise, like the previous two cascades. It descends instead— more effectively, to my ear— like a cascade in nature, a skein of waterfalls, say. Its effectiveness is enhanced by what it descends from: a high A. This isn’t just the highest note in the piece, but the highest note Bach ever asks the human voice (at least the choral human voice) to produce. The sopranos’ octave leap to this A makes the note all the more electrifying. That this A isn’t just stratospheric but syncopated— a trait that comes down to it all the way from the germ of the cascade motive in the Kyrie’s third measure—heightens its impact further still.

I haven’t yet mentioned the most notable thing about this cascade, Bach’s tiny yet radical modification of the cascade motive. This modification has its origin in the Kyrie’s subject, specifically, in the subject’s signature interval of a diminished third: