The Birth of Polyphony at Notre-Dame

What happened

At the cathedral of Notre-Dame in Paris, composers began layering independent melodic lines over the old plainchant. Léonin and Pérotin, the leading figures of the 'Notre-Dame school,' developed organum — music for two, three, and four simultaneous voices — turning single-line chant into rich, interweaving polyphony that filled the great stone vaults with sound.

Why it matters

Polyphony — many voices sounding at once — became the defining feature of Western music, setting it apart from most other traditions. The Notre-Dame composers took the decisive first steps toward the harmony and counterpoint that would culminate in Bach.

Sources

• Medievalists.net. Why Notre-Dame Was Built for Medieval Music · Reputable source