Volvelle
A revolving wheel or wheels of parchment or paper (often within a book or attached to a bookmarker) that carries information of a computational, astronomical, or astrological character.
A revolving wheel or wheels of parchment or paper (often within a book or attached to a bookmarker) that carries information of a computational, astronomical, or astrological character.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).