This chapter focuses on the cantus-firmus mass, the genre's predominant type, embodied and expressed the key needs of its patrons, in terms of both spiritual welfare and public show, personal and political. The polyphonic setting, dating back to the earliest stages of polyphony, of liturgical chants continues throughout the fifteenth century. The structural principle of the "cyclic" mass based on a given cantus firmus clearly grew out of the motet-based practice of isorhythm. The tenor line, monolithically repeating in contour and rhythm from movement to movement, provides melodic material for the other voices and a rhythmic check on their progress in fully-scored sections. L'homme arme song was by far the most popular and probably the most ingeniously adapted cantus firmus of the fifteenth and early sixteenth century. A seemingly gentler spirit in comparison to the more Dionysiac Antoine Busnoys, Johannes Ockeghem seems to have reinvented his approach to the Ordinary of the Mass with each setting.