In cryptography, the ADFGVX cipher was a manually applied field cipher used by the Imperial German Army during World War I. It was used to transmit messages secretly using wireless telegraphy. ADFGVX was in fact an extension of an earlier cipher called ADFGX which was first used on 1 March 1918 on the German Western Front. ADFGVX was applied from 1 June 1918 on both the Western Front and Eastern Front.

The cipher is named after the 6 possible letters used in the ciphertext: A, D, F, G, V and X. The letters were chosen deliberately because they are very different from one another in the Morse code. That reduced the possibility of operator error. [Wikipedia]