Zooming in on a single city, Chapter 4 focuses on Paris in the twelfth and thirteenth centuries. As a prime example of the social changes brought about by urbanization, Paris was a commercial hub, the seat of royal administration, and a centre for advanced learning and education through its new university. We explore how this environment fed into the cultivation of polyphonic music at the Cathedral of Notre-Dame, looking into the genres of organum, conductus, and motet, and examining the system of rhythmic modes that was developed to notate this music for posterity. Music theorists, writing a generation or more later, provide us with the names of some of those musicians responsible for the musical innovations at Notre-Dame, and thus we can identify the composers of liturgical polyphony for the first time. We learn how Léonin compiled his Magnus Liber Organi, and his successor Pérotin edited and supplemented it, giving us a unique insight into the ways in which medieval musicians preserved and reinvented the music of earlier generations.