Polyphonic Settings
Graduals were among the parts of the Mass most frequently composed as organa, including both the St. Martial School and the Notre Dame School. Ordinarily the parts that were sung by the soloist (the beginning of the respond and the verse) are the only parts so set, while the choral parts continued to be performed in plainsong. In 1198, Odo de Sully, Bishop of Paris, authorized polyphonic performances of Graduals, including Pérotin's famous four-part organa, Sederunt principes for St. Stephen's Day and Viderunt omnes for Christmas.
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