Bookbinders need to trim the corners of book cloth on covers. These jigs are designed to assist with that. Snug them up against the binder's board, then use the angled side as a straightedge to trim the cloth using a razor. The resulting cut is a perfect 45 degree angle with regard to the board, spaced either 2, 3 or 4 mm away from the corner depending on which jig you use.

The printed jigs shown in the illustration are an earlier revision. The STL files here are 2 cm longer to allow for wider turn-ins, and have a 6 mm hole in one end so they can be hung on a peg for storage.