Today's Music Theorist of the Day is Heinrich Glarean (1488-1563)!
Glarean, though, began to think of the octave (diapson) as a unit instead of the sum of a fourth and a fifth. This insight, coupled with a dire misunderstanding of Greek modal theory* and (panicked) Catholic thinking during the Reformation** led him to posit a system of 12 (!) modes in his Dodecachordon (1547).
In his taxonomy, every diatonic note except B (owing to the false fifth) could be a final, with corresponding authentic and plagal modes for each. Thusly the number of modes leapt dramatically in his conception from 8 to 12 (!), which added the now-familiar Ioninian and Aeolian modes known to today's music students.
I knew you were wondering.
**SUPER HAPPY BONUS MODAL THEORY MNEMONIC!**
"But, Sator Arepo," you're thinking, "how will I ever memorize up to and including seven Latin words?"
Today we know the "church" modes and their finals (ending notes) including the more-or-less never-used Locrian mode as:
So just remember this:
Thanks, conservatory education!
* See Palisca, "Mode Ethos in the Renaissance" in Essays in Musicology, ed. Lockwood, pp. 126-39 (1990) .
** See Judd, "Renaissance Modal Theory" in Cambridge History of Western Music Theory, ed. Christiansen, p. 388, Cambridge University Press (2002).