ADFGX cipher
A German field cipher from World War I that combined a Polybius square with a columnar transposition.
In the spring of 1918 the German army used the ADFGX cipher in its offensives. The name comes from the five letters used as coordinates in a 5×5 grid, which replaced each plaintext letter with a pair of ciphertext letters. The letters were chosen because they are easy to tell apart in Morse code, which mattered when messages went out by radio and telegraph.
The second stage was a columnar transposition keyed by a word. That was enough to stop many readers, but not Georges Painvin, who broke the system in June 1918. A later version added the letter V and became ADFGVX.