In the Vatican Library sits a manuscript that liturgical scholars treat the way biblical scholars treat their oldest codices: Barberini graecus 336, the Barberini Euchologion, copied in southern Italy in the second half of the eighth century and the oldest surviving service book of the Byzantine rite. Its survival is an accident of geography; southern Italy was then Greek in rite, and books written there escaped the losses that consumed so much in the East.
What the manuscript preserves is the Constantinopolitan liturgy at a stage centuries earlier than any other complete witness: the anaphoras of Basil and of John Chrysostom in early forms, orders of baptism, ordination prayers, the offices of the cathedral rite of the Great Church, and dozens of occasional prayers, some of which later books dropped.
Scholars mine it for the history beneath the received text, where it shows, among much else, that the liturgy attributed to Basil then held the place of honor that Chrysostom's liturgy holds today. The modern critical edition by Stefano Parenti and Elena Velkovska, published in 1995, made the manuscript fully usable and is now the standard reference. The codex anchors a simple point here: the Euchologion is not a modern compilation but a book with a documented thirteen-century manuscript history, and the prayers a Byzantine priest reads tonight can be checked, line against line, with what a Calabrian scribe wrote before Charlemagne was crowned.