In 1930, the German army introduced an additional security feature, a plugboard ("Steckerbrett" in German; each plug is a "Stecker", and the British cryptologists also used the word) that further scrambled the letters. The Enigma encryption is a self-inverse function, meaning that it substitutes letters reciprocally: if codice_2 is transformed into codice_3, then codice_3 is transformed into codice_2. The plugboard transformation maintained the self-inverse quality, but the plugboard wiring, unlike the rotor positions, does not change during the encryption. This regularity was exploited by Welchman's "diagonal board" enhancement to the bombe, which vastly increased its efficiency. With six plug leads in use (leaving 14 letters "unsteckered"), there were 100,391,791,500 possible ways of setting up the plugboard.