The historical development I have traced in this book is marked by both change and continuity. Change is of course to be expected in a trajectory that extends for well over a millennium. From its formative period in late antiquity, the ritual shape of the Roman Mass was affected by many religious, social, cultural, political and economic transformations. It is the essential continuity that should be noticed. The celebration of the Eucharist as a liturgical act is rooted in the words and actions of Jesus at the Last Supper. The priestly and sacrificial character of this liturgical act is clear even in the early Christian period, when sources are few and far between. The Latin liturgical tradition becomes more tangible to us from the fourth century onwards, above all with the early form of the Canon of the Mass attested by Ambrose. The ritual structure of the Roman Mass was forged in the practice of the papal stational liturgy of the late ancient and early medieval periods. Many sacramentaries of the Gregorian type begin with a separate section ‘How the Roman Mass is to be celebrated’, which corresponds to the description of Ordo Romanus I.