What is the musical style of Notre Dame School in Gaude Maria Virgo?

The musical style of the Notre Dame School's *Gaude Maria Virgo* is organum. More specifically, it's an example of early polyphony, a style of music that uses two or more independent melodic lines simultaneously. Within organum, this particular piece would be classified as parallel organum or possibly organum purum, as the voices often move in parallel motion at intervals of a perfect fourth or fifth.