The sacramentary is a book for use by the celebrant at Mass and therefore it contains the common and proper texts and chants that he intoned, read, or sang. Other parts of the Mass can be indicated by their incipit, and frequently no musical notation is provided except for what the priest sang. By the end of the thirteenth century, the sacramentary and other books for the celebration of the Mass (the lectionary for the Mass and the gradual) had been combined into a single book, the missal. A missal combines in a single volume what had previously been separate books for the celebration of Mass, including the sacramentary, the gradual, cantatorium, and lectionary or Gospel Book and Epistolary.
Christianity, as many other religions, relies on the Word, a written text - in this case, the Bible. Reading the Bible, therefore, is the focus of the first part of the Mass. During the Middle Ages, the passages excerpted varied in number and selection; the list of readings sometimes even provides hints as to the origin of a given missal. In the Ambrosian rite, for example, as still celebrated in the diocese of Milan, there are three readings at Mass (including one that could be from a saint’s life); much more common were two readings, one from the Epistles, read by the subdeacon, and one from the Gospels, read by the deacon. Gospel lectionaries are easily recognized by the words that introduce each passage, “In illo tempore . . .”, “At that time . . ..” Since books were expensive, a cleric might simply annotate the margins of his copy of the Bible with the days for certain readings, rather than buy a separate lectionary for the Mass.