Mors is a discant clausula for four voices, attributed to the medieval French composer Pérotin (active ca.1200), one of the greatest composers of the early polyphony known as organum. Mors is a clausula attached to Alleluia V. Cristus resurgens, the Alleluia chant for the Mass and first vespers of the Wednesday in Easter week, and for the Mass of the Sunday before Ascension. When Pérotin revised the collection of organa known as the Magnus liber organi, he added and replaced certain sections created by earlier composers. These replacements were called clausulae. This clausula, consisting only of the word "Mors", is further defined as a discant clausula, for it is in the discant or note-against-note style of composition. It is the only known clausula for four voices. There are, however, no corresponding extant four-voice settings of Alleluia V. Cristus resurgens itself; the closest is a three-voice setting of which we have only a fragment. ~ David Cashman, All Music Guide
