source: https://en.wikipedia.org/wiki/ADFGVX_cipher
In cryptography, the ADFGVX cipher was a field cipher used by the German Army on the Western Front during World War I. ADFGVX was in fact an extension of an earlier cipher called ADFGX.
Invented by Lieutenant[1] Fritz Nebel (1891–1977)[2] and introduced in March 1918, the cipher was 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. Letters were chosen deliberately because they are very different from one another in the Morse code. That reduced the possibility of operator error.
Nebel designed the cipher to provide an army on the move with encryption that was more convenient than trench codes but was still secure.
In fact, the German believed the ADFGVX cipher was unbreakable