The cipher disk is a deceptively simple cryptographic tool invented around 1470 by an Italian architect. It consists of two concentric disks marked with letters, numbers, and other symbols around the edge of each disk. The smaller disk, which is mounted on the stationary larger disk, can be moved to create a cryptographic key. The appeal of the cipher disk lies in the fact that messages can be enciphered and deciphered without the need for bulky or compromising written materials.
The cipher disk first came into large-scale use in the United States during the Civil War. About a half-century later, the U.S. Army adopted a version of the device, which used both a standard and a "reverse-standard" alphabet. The two disks may be left in the same setting to create an entire message, thereby producing the simplest possible cryptogram, or the setting may be changed with every letter of the message to create an extremely secure cipher.