These odd-looking medieval books share one peculiarity: they were all made into interactive objects because actual turning discs were attached to the page, usually more than one. The makers of these manuscripts added them to calculate the position of sun and moon (Pic 1), the date of Easter (not shown), or make other calculations (Pic 3). Particularly intriguing is the set of cogwheels embedded in the bookbinding (Pic 2), which picked a random number used for a method of divination. More about these unusual books and their function in this post on my other blog, medievalbooks.nl.
Pics: British Library, Egerton 848 (top); Oxford, Bodleian Library, Digby 46 (middle); Maastricht, Regionaal Historisch Centrum (bottom, pic my own).