This document summarizes the development of polyphony from its origins in organum through the Notre Dame school in the late 12th/early 13th centuries. It discusses key figures like Léonin and Perotin who expanded organum to include more voices with intricate rhythms. Other developments discussed include clausulae, motets which added secular texts, and conductus which were freely composed sacred songs. The document concludes by introducing mensural notation which developed in the late 13th century to notate the increasingly complex rhythms of polyphonic music.