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Rotating bookmarks were a special kind of bookmark used in medieval Europe. They were attached to a string, along which a marker could be slid up and down to mark a precise level on the page. Attached to the marker was a rotating disk that could indicate the column (usually numbered one to four, indicating the two columns on the left-hand page, and the two columns on the right-hand page).
About 30 such rotating bookmarks have been recorded in libraries in continental Europe, and another half a dozen in England.
References
Rotating bookmark Wikipedia(Text) CC BY-SA