A metal fitting attached to the boards at the fore edge of a binding in order to hold the book shut and to preserve the parchment (unless kept at an appropriate temperature and humidity level, parchment tends to cockle and return to the original shape of the animal skin). Clasps became popular during the fourteenth century (alongside their earlier counterpart, the strap and pin), initially as a combination of metal fittings and leather straps and then entirely of metal. On English and some French bindings the clasps fasten at the lower board, while elsewhere on the Continent the catch is on the upper board.
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).