In 1198 Pope Innocent III initiated a regularized system of record keeping at the Lateran Palace Library in which copies of letters sent were entered by hand in great registers. These were called the Vatican Registers.
"This series is one of the principal sources for documents on the papacy between the years 850 and the reorganization of the papacy in 1588. From the perspective of the history of the nature of documentation, the Vatican Registers are important in that they were regular in format and durable" (Blouin, Jr. Vatican Archives: An Inventory and Guide to Historical Documents of the Holy See [1998] xviii).