Regular radio transmissions of "Tunny" messages started in June 1941. The British codebreakers saw that it used a five-unit code like a teleprinter system. Their research also showed that the coding was being done by a rotor cipher machine with 12 wheels (rotors). For each new message sent, the wheels had first to be turned to new positions. The start position for the message was chosen by the operator who was sending the message. He told the operator who was receiving the message what the start positions were with 12 letters that were not coded. The total number of possible start positions of the 12 wheels was very large indeed.