The Jefferson disk, also called the Bazeries Cylinder or wheel cypher as named by Thomas Jefferson, 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 ordered randomly. Each disk is marked with a unique number and a hole in the center 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. Once the disks have been placed on the axle in the agreed order, the sender rotates each disk up and down until a desired message is spelled out in one row. Then, the sender can copy down any row of text on the disks other than the one that contains the plaintext message. The recipient has to arrange the disks in the agreed-upon order, rotate the disks so they spell out the encrypted message on one row, and then look around the rows until they see the plaintext message, i.e. the row that's not unreadable. There is an extremely small chance that there will be two readable messages, but this can be easily avoided by checking. The Jefferson disk was invented by Thomas Jefferson in 1795, and the same method was independently invented and improved upon by Commandant Etienne Bazeries, but did not become well known until he broke the Great Cipher, of Rossignols, a century after Jefferson's work. The system was used by the United States Army from 1923 until 1942 as the M-94. It aided the war efforts as it was a type of secret messaging that only military and government officials would know how to interpret. Example of Operation To encrypt a message, the encrypter rotates the disks to produce the plaintext message along one "row" of the stack of disks, and then selects another row as the ciphertext. To decrypt the message, the decrypter rotates the disks on his cylinder to produce the ciphertext along a row. Decryption is easier if both the encrypter and the decrypter know the offset of the row, but not necessary since the decrypter can look around the cylinder