Show off your theory chops with my weekly challenge! You’ll find a new question here every Monday. Please comment to post your reply.
This week’s challenge:
Below is the Kyrie from the Missa Prolationum, a mass by Renaissance composer Johannes Ockeghem. These are just two of the parts, but it’s enough to see what rhythmic principle Ockeghem based the piece on. What do you notice about the parts, and what’s the name of this compositional method?

Check back on Friday, January 28th to see if you’re right!
ANSWER for 1/24/22
This is a mensuration canon, also called a prolation canon. As in an ordinary canon, both voices pictured above have the same melody, but the bottom voice’s note values are one and a half times longer than those of the top voice. The result is that the bottom melody is really in 3/4, and “stretches away” from the top melody as time goes on. It’s a very tricky type of canon to write well.
But wait! Why have just one mensuration canon at a time when you can have two! The Missa Prolationem is a truly astounding setting of the Ordinary of the Mass because it’s based entirely on double prolation canons. In the original manuscript below, each page features a different melody. You can see the mensuration signs circled in red, which were archaic time signatures in the late Medieval period and Renaissance. Notice that each page has TWO of these signs, which means that each melody is to be sung by two voices, each with a different time signature. The melody on the left page is sung by one voice in simple triple meter, and another in simple duple. Not complex enough yet? At the same time that this is going on, a third voice sings a different melody on the other page in compound triple meter, and a fourth voice sings it in compound duple. So it’s really one mensuration canon in counterpoint with another! For the modern rendition I pictured above, I only showed what was happening on the first page.

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