I’ve never heard it described as symmetrical, but it is a common understanding in early music that it follows the “tactus,” a steady pulse roughly 60 bpm or the average heart rate (wish mine was 60!). There is to be no ritardandi or rubato in this music - which is why to indicate closure at cadences, they often feature longer note values or elaborations against a sustained tone.

Personally, I think this is a boring way to sing...

Madame,

I would have thought that the question to ask was this: at the Alleluia, is there a pause before singing the 3/2 section, or isn't there. (To put a pause where the piece reverts from 3/2 to 4/4 is silly, so why not go seamlessly into that same 3/2 section?). To say (if I understand the claim that was made to you) that "O Great Mystery" should be sung in an identical manner to "Alleluia" is unfortunate.

To pick up on Schoenbergian's point, imagine someone making the point that the repeated section in Byrd's Ave Verum had to be sung at exactly the same volume level on the repeat as on the original pass...…, or in Vittoria's Ave Maria, that the repeated sections must be sung identically.

PIP input delivered with some heat - that we are singing de Victoria's "O Magnum Mysteriumc do not participate in the music"

There is a certain place for musical experts who sit in the pews who do not participate in the music of the parish but do pontificate.

I'm sure there are many interesting ways that one goes from the 4/2 to 3/2, and back again.
Is there already a thread about this? It does seem to me that many people just go ahead and choose whatever tempo they prefer for the 3/2, rather than basing it on something technical.

If - hypothetically speaking - you were to conduct this "in 4," would your quarters stay constant throughout the entire work?
If not, do you try to keep the length of the measure the same, from the 4/2 into the 3/2?
If neither of those, on what basis do you direct your choir to sing the 3/2 section?

You might find this video on tactus and proportions interesting. (Peter Kwasniewski did an interview with Elam Rotem at NLM a few weeks ago.)
https://youtu.be/otgXoa8QEWg

What I have always done, and what seems most natural to me, is that each measure of "3" is equal to one beat in "2". So the pulse stays steady, but there is a significant speed-up.

Is that kosher?

As a modern, albeit very traditionally composed, example of the Sesquialtera 3:2 proportion, there is my own 6 part Ave verum corpus, newly tweaked for future performance, in which the 6/2 "triple proportion" Section C ("esto nobis praegustatum in mortis examine") begins at m.29 (1m52s into the recording). The rest of the composition is in 4/2 "duple proportion". Note the tempo markings are minim=60 in the 4/2 sections and minim=90 in the 6/2 sections.

As a side note, Section B at m.19 (1m12s into the recording) is a prolongation canon between the highest and lowest voices, with a countermelody in the middle voices ("cuius latus perforatum, unda fluxit sanguine").

Oh, wow! Schönbergian, thank you very much for your kind words. A little aside, however: not all of my works are traditionally composed, but the Ave verum corpus definitely represents my great admiration for Byrd and Tallis, as well as the deep meaning of the text.

That indeed is how that the polyphonic tradition is a living art whose modern proponents bring an ever-fresh approach to it. Even in the golden age of polyphony there was a great variety of dialect in the repertorial corpus. That is why Victoria is not Palestrina and could not be confused with him. In our day there are many gifted composers who write in the antique style. Yet, their work is recognisably modern. Some have a textural complexity that would rival that of Tallis or Byrd, or even Taverner. Others (e.g. Alan Hovhannes) have a very sparse and clean texture which is unmistakably modern while being representative of its unmistakable pedigree.

Too, we moderns are not alone in our custodianship of the stile antico. It is a commonplace amongst musicologists and music historians that the English were, well into the XVIIth century, writing music which was 'out of date' and 'old fashioned'. Yet their music could not at all be confused with the polyphony of the previous century.

Beautiful work, Chuck.
Keep it coming!
Your geneology is of the rarest.

And we can be thankful that it did. Else we would not have the exquisite work of Weelkes, Tomkins, Gibbons, Jenkins, et al. Most English music, excepting that of Purcell, that followed the Inter-regnum is of little if any consequence: pale and shallow imitations of an evolving, new-born continental art. The English never excelled at the stile moderno.

To bring this discussion back to symmetry and the question of a different tempo at the end of a Renaissance piece than at the beginning, here is yet another O magnum mysterium, this time by Palestrina, which has a different structure from the Victoria. In particular it has a Prima Pars and a Seconda Pars, each part having the text 'et choros angelorum collaudantes Dominum: Alleluia." The "collaudantes Dominum" is in triple time (Sesquialtera). Incidentally, the animated scrolling score used is from my edition at CPDL.