Pérotin, active around the late 12th and early 13th centuries, was a pioneering French composer associated with the Notre-Dame school in Paris. He is renowned for advancing polyphonic music by introducing three and four-part harmonies, a significant evolution from the two-part compositions of his predecessor, Léonin. Pérotin's notable works include "Viderunt omnes" and "Sederunt principes," both exemplifying his innovative approach to organum. His contributions laid the groundwork for the development of Western polyphony, influencing generations of composers and shaping the course of medieval music. ([britannica.com](https://www.britannica.com/biography/Perotin?utm_source=openai))