Ciphering device comprising two superposed disks of different diameters. Each disk carries three concentric rings divided into twenty-four cells containing the letters of the alphabet. The larger, fixed disk carries a suspension ring; the smaller, revolving disk has a small index on its rim. At the center is hinged an index arm that can rotate on both disks. The instrument made it possible to develop coded languages through the coordinated substitution of the letters of the alphabet shown by the index arm and the index of the smaller disk. Provenance: Vincenzo Viviani bequest.