A protruding ornament (on a binding), usually of metal. When applied to a binding it serves a protective function. Metalwork plaques are known to have adorned bindings from the Early Christian period on, but prominent raised bosses appear to have become popular during the fifteenth century.
Michelle Brown , mzh
Boulon; Ombilic (center boss); Bossette; Bouillon; Cabochon
Beschlag (Mittelbeschlag, Eckbeschlag)
Borchia; Borchia centrale; Bullone; Brocca; Coppa; Bulletta
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).