By about the middle of the 4th century there were certainly some liturgical books, How long before that anything was written one cannot say. The first part of the liturgy to have been written appears to have been the Diptychs. The word Diptych is derived from the Greek for twice­folded. A Diptych consisted of two tablets (covered with wax at the beginning) hinged and folded together like a book. On one the names of the living for whom prayers were to be said were written, on the other the names of the dead. These names were then read out by the deacon at the appointed place in the liturgy. Their use, in the East went on till far into the middle ages. Then the lessons were set down in a book. The old custom of reading from the Bible until the bishop made a sign to stop, soon gave way to a more orderly plan of reading a certain fixed amount at each liturgy. Marginal notes were added to the Bible showing this. Then an Index giving the first and last words of the amount to be read is drawn up. Other books were read besides the Bible (lives of Saints and homilies in the divine office); a complete Index giving references for the readings is the "Companion to the books"­ comes, liber comitis or comicus. Lastly, to save trouble, the whole texts are written out as they are wanted, so we come to the (liturgical) Gospel­book (evangelarium), Epistle­book (epistolarium), and finally the complete Lectionary (lectionarium). St. Jerome (324-420) is widely believed to have been commissioned by the pope to select the Epistles and Gospels used for each Sunday of the liturgical year, which have been used since in the traditional Roman Missal. Meanwhile the prayers said by the celebrant and deacon are written out too.

Here we must notice an important difference between the older arrangement and the one we have now in the West. Our present books are arranged according to the service at which they are used; thus the Missal contains all that is wanted for Mass, the Breviary contains all the Divine Office, and so on. The older system, still kept in all Eastern churches, considers not the service but the person who uses the book. One book contained all that the bishop or priest says at any service, the deacon has his book, the choir theirs, and so on. The bishop's book, from which the priest also used whatever he needed is the Sacramentary (Sacramentarium or liber sacramentorum). It contained only the celebrant's part of the Eucharistic liturgy,such prayers as the Canon, Collects, and Prefaces, but not the Epistles and Gospels or such sung parts as the Gradual. It also contained the bishop's part in many other services, ordinations, baptism, blessings and exorcisms,in short all sacerdotal functions. The deacon had his book too, the diakonikon; but as his function at Rome was reduced to singing the Gospel this book was confined to the Eastern liturgies. And then, later, the choir had the psalms and responses arranged together in the liber antiphonarius or gradualis, the liber responsalis, psalterium; later still the hymnarium, liber sequentialis, responsalis, and the psalterium6, later still the hymnarium, liber sequentialis, and so on, of which in the early middle ages there was a great variety.