The Jefferson disk, or wheel cypher as Thomas Jefferson named it, also known as the Bazeries Cylinder, is a cipher system using a set of wheels or disks, each with the 26 letters of the alphabet arranged around their edge. The order of the letters is different for each disk and is usually scrambled in some random way. Each disk is marked with a unique number. A hole in the centre of the disks allows them to be stacked on an axle. The disks are removable and can be mounted on the axle in any order desired. The order of the disks is the cipher key, and both sender and receiver must arrange the disks in the same predefined order. Jefferson's device had 36 disks. [Kahn, p. 194]
First invented by Thomas Jefferson in 1795, this cipher did not become well known and was independently invented by Commandant Etienne Bazeries, the conqueror of the Great Cipher, a century later. The system was used by the United States Army from 1923 until 1942 as the M-94.