The Germans had specific regulations regarding which kinds of codes and ciphers could be used under given circumstances. Within three kilometers of the front lines, known as the danger zone, all communications were required to be in a code known as the three-number code. This was the only code or cipher permitted. Behind this danger zone, another code known as the three-letter code was allowed to be used. Communications between divisions, corps, and army headquarters were done with the ADFGVX cipher.