The ADFGVX cipher was a field cipher used by the German Army during World War I. It is closely related to the ADFGX cipher. ADFGVX is a fractionating transposition cipher which combined a modified Polybius square with a single columnar transposition. The cipher is named after the six possible letters used in the ciphertext: A, D, F, G, V and X. These letters were chosen deliberately because they sound very different from each other when transmitted via morse code. The intention was to reduce the possibility of operator error.