In the 12th century, polyphonic music continued to develop through the Notre Dame School in Paris. Composers such as Léonin and Pérotin were active between 1160/80 and 1230/50. Léonin created the important Magnus liber organi, a collection of two-part works for church celebrations. Pérotin later revised these compositions. Both musicians worked at Notre Dame Cathedral in Paris.
The Magnus liber organi contained polyphonic chants in various styles and required the participation of all voices in the performance. The group Red Byrd interprets various polyphonic works from this period, including Alleluia compositions for feast days such as Easter, Pentecost, and the Assumption of Mary. Authentically performing the duplum in organum presents a particular challenge today, as interpretations have changed over time.
The organum duplum was characterized by long sustained notes. More recent interpretations emphasize the use of rests. The musical texts used for recordings come from manuscripts in Florence and Paris and belong to the Magnus liber organi de Notre-Dame de Paris.