▁form ▁of ▁poly phony ▁of ▁the ▁period . ▁P é rot in ' s ▁two ▁Grad uals ▁for ▁the ▁Christmas ▁season ▁represent ▁the ▁highest ▁point ▁of ▁his ▁style , ▁with ▁a ▁large ▁scale ▁t onal ▁design ▁in ▁which ▁the ▁massive ▁ped al ▁points ▁sust ain ▁the ▁sw ings ▁between ▁consecutive ▁harm on ies , ▁and ▁an ▁intr icate ▁inter play ▁among ▁the ▁three ▁upper ▁voices . ▁P é rot in ▁also ▁further ed ▁the ▁development ▁of ▁musical ▁notation , ▁moving ▁it ▁further ▁from ▁impro vis ation . ▁Despite ▁this , ▁we ▁know ▁nothing ▁of ▁how ▁these ▁works ▁came ▁about . ▁ ▁In ▁addition ▁to ▁his ▁own ▁compos itions , ▁as ▁noted ▁by ▁An onymous ▁IV , ▁P é rot in ▁set ▁about ▁revis ing ▁the ▁Magn us ▁Liber ▁Or g ani . ▁Lé on in ' s ▁added ▁du pl um ▁required ▁skill , ▁and ▁had ▁to ▁be ▁s ung ▁fast ▁with ▁up ▁to ▁ 4 0 ▁notes ▁to ▁one ▁of ▁the ▁underlying ▁chant , ▁as ▁a ▁result ▁of ▁which ▁the ▁actual ▁text ▁progress ed ▁very ▁slowly . ▁P é rot in ▁short ened ▁these ▁pass ages , ▁while ▁adding ▁further ▁voice ▁parts ▁to ▁en rich ▁the ▁harm ony . ▁The ▁degree ▁to ▁which ▁he ▁did ▁this ▁has ▁been ▁deb ated ▁due ▁to ▁the ▁phrase ▁abbre via v it ▁e und em ▁by ▁An onymous ▁IV . ▁Usually ▁translated ▁as ▁abbre vi ate , ▁it ▁has ▁been ▁sur m ised ▁that ▁he ▁short ened ▁the ▁Magn us ▁liber ▁by ▁replacing ▁organ um ▁pur