The Magnus Liber Organi, or "Great Book of Organum", is the largest and most important source for the influential composers centered around Notre Dame cathedral in Paris. Compiled during the 12th and 13th centuries, its most noted contributors included Léonin and his successor Pérotin. This work represents a step in the evolution of Western music between plainsong and the intricate polyphony of the later 13th and 14th centuries. The music of the Magnus Liber was used in the liturgy of the church throughout the feasts of the church.
Biblical Heritage Gallery, Cedarville University, Magnus Liber Organi