A Medieval Revolving Bookmark, manuscript on vellum
The upper end of the string would be attached to a manuscript’s headband. It could be located between any two pages, and the marker could be slid up or down the string to the exact level to be marked, and the disc would then be turned to indicate column 1 or 2 on the left-hand of the opening or 3 or 4 on the right. About 30 such rotating bookmarks have been recorded in continental libraries (cf. J. Destrez, ‘L’outillage des copistes du XIIIe et du XIVe siècles’, Aus der Geisteswelt des Mittelalters, Martin Grabmann festschrift, 1935, pp.19-34) and about half a dozen from England (cf. R. Emms, ‘Medieval Rotating Column-Indicators’, Transactions of the Cambridge Bibliographical Society, XII, 2001, pp.179-84).