A protective support at the sewing edge of a manuscript. During the Middle Ages, parchment guards were sometimes folded around the spinal edge of a quire or bifolium to strengthen it, especially in early paper manuscripts. In the process of modern rebinding, leaves are often mounted on guards to protect them and to reveal the maximum amount of codicological information.
Tallone, Brachetta; Braghetta
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).