I've been reading up on the music theory behind Gregorian Chant, and it is fascinating.

The music style developed as vocal ornamentation for texts of Scripture. The inflections in the music resemble the inflections of a good orator who lets his voice rise and fall for effect at various points within a sentence.

Over time, 8 modes emerged. Each mode conveys a different emotion—solemnity, reverence, joy, and so on—and each revolves around two primary pitches. There is the tonic, the pitch the chant ends on, and the dominant, either a perfect fifth or a minor third above the tonic, that contains the center of the melody's motion.

Thus, with a few elements, the medieval monks developed an oral tradition that allowed them to memorize the pitches and basic melodic elements used in each mode, which aided them in chanting the Scriptures, especially the Psalms, in communal worship.

Over time the tradition grew and was elaborated into the sacred music corpus that has been handed down to the Church today.