Oy... medieval music theory is such a huge subject that I couldn't begin to do it justice in a HN comment. The short version is that some time in the 9th centur

Oy... medieval music theory is such a huge subject that I couldn't begin to do it justice in a HN comment. The short version is that some time in the 9th century monks developed neumes, an early form of musical notation, to write down the chants they chanted at different times of the day. Medieval music theorists classified those chants according to the last note (what we now call the "tonic") and the most common note (what we now call the "dominant"). There were 262144 theoretically possible melodies (8 notes long, 6 possible notes, but some were rejected for music-theoretic reasons so the actual space was rather smaller) which could be indexed by their sequence in a collated list of melodies ("on the Sunday after Pentacost sing the 7th chant in the Lydian mode" or whatever). If I had to pick a "best-of" reading list, I'd say the 9th-century "Musica Enchiriadis", Berno's "Prologus in Tonarum", and Boethius's "De Institutione Musica". These all assume a familiarity with Platonic music theory, so at least skim through Plato's "Timaeus" first. There will also be more references to astronomy than will make sense to somebody in the modern age, because music and astronomy were believed to be essentially the same thing. Also more recent research has found that actual musicians of the time seem to have completely ignored the theorists. Some things never change.