main printed feature: register

Early printers sometimes included a statement at the end of the book giving the order of the gatherings so that the binder could be sure they had the complete book in order. These statements are known as registers, and their use generally faded away over the course of the hand-press period.

Boethius, Arithmetica, 1492 (2l8r)

On this last leaf of text, the printer has included both a colophon identifying who printed the book and a register of the last words printed on the first four leaves in each gathering of the two-volume set.

Erasmus, Novum Instrumentum, 1516 (Ff8r)

The last leaf of this work includes a colophon and printer's device as well as a register for the order of the book's gatherings. You can see that the preliminaries were printed after the rest of the work by how they're signed (aaa, bbb); you can also note that there are two series of double letters and that AA is not the same as Aa.