Each Enigma I machine was equipped with 12 of these cables (although the plugboard could hold up to 13). Each cable, 20cm long, had a black bakelite plug at each end with one thick pin on the top (measuring 4mm) and one thin pin below it (measuring 3mm). Connecting the two pins were two wires - one connecting the thick pin on one plug to the thin pin on the other, and vice versa. The plugboard has 26 sockets, one for each of the letters A-Z. Each socket has two contact. The sockets of the plugboard are self-closing, meaning that if no plug is inserted, the two contacts are automatically shorted by an internal spring-loaded shorting bar (attached to the end of the socket, protruding behind the plugboard). The cable is used to swap pairs of letters, making it self-reciprocal. If any number of cables (0≤N≤13) had been used, the number of combinations would have been 532,985,208,200,000. In reality, however, six, and then later 10 cables were used at a time, reducing the number of possibilities. In the latter case, the number of combinations would be 'only' 150,738,274,900,000.