Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

fillet

1. A wheel-shaped finishing tool having one or more raised bands on its circumference. It is used to impress a line or parallel lines on the covering material of a book, usually one bound in leather. The lines may be continuous or the fillets may have a wedge-shaped gap in the circumference to facilitate starting and stopping lines and also to enable lines to be joined evenly at corners.

It is not known when the fillet first came into use. Bindings of the 12th century, and even earlier, have impressed lines that could have been made with a fillet, but they may also have been impressed with a pallet, or similar tool, dragged across the leather rather than rolled. It is argued that it probably did not precede the roll, which was introduced in about 1470. by any great length of time, because once a wheel-type tool was introduced, it would soon be patterned. It is sometimes called a "roulette" in the United States.

2. The plain line or lines impressed on a book cover. The so-called French fillet is a triple fillet (always in gold) having unevenly spaced lines. (69 , 161. 172 , 236 , 335 )