Pérotin worked on a very important collection of polyphonic music called "Magnus Liber" (meaning: Big Book). The composer Léonin had written a lot of this music. He may have been Pérotin's teacher, but we do not know. He seems to have written music for four voices which are settings of words for the Feast of the Circumcision and St Stephen's Day. He wrote three and four voice music of a type called organum. It was a very important development in the history of Western music. He wrote music in a polyphonic style. Often he took a tune and arranged it so that one voice sang it incredibly slowly while another voice sang something much faster on top.