During the period of Notre Dame polyphony, around 1180-1250, it was common to have a polyphonic organum composed by a single composer over a plainchant tenor with new composed voice or voices. Leoninus and Perotinus were notable for organizing this music into sections based upon rhythmic modes. This musical style became an important innovation, giving the music greater rhythmic freedom and expression. Leoninus composed these melodies in the Magnus liber organi (“Great Book of Organum”), which later became expanded upon by Pérotin. This manuscript is one of the most celebrated collections of polyphony from this period.
Pérotin contributed greatly to the development of more complex polyphonic music. In addition to adding additional voices to the organum, he experimented with different textures and forms, adding extra notes and melismas to the existing melodies, and incorporating other techniques such as syncopation and hocketus, making the music more engaging and dramatic. He is said to have composed four-voice organa quadruplum. Two- and three-voice organa can be found in the Magnus Liber. However, this manuscript does not contain any three- or four-voice organa that can with certainty be attributed to Perotinus. His works include Sederunt principes, Alleluia Nativitas and Viderunt omnes.
Their contributions to the development of polyphony and their innovative use of rhythmic modes were influential in shaping the course of Western music, helping to establish the foundation of Western polyphonic music composition.