clasp
OAB Definition
metal or leather fixture, usually on medieval or early modern books, that hold a book firmly closed.
Clasps were important for books with parchment or vellum pages, which are easily affected by changes in humidity. Clasps can also protect books with paper pages, but they fell out of common use in the 16th century because of the transition from wooden book boards to paste paper, to which the clasps cannot be well-secured, in addition to other reasons.