A few million reed relays were used from the 1930s to the 1960s for memory functions in Bell System electromechanical telephone exchanges.[2] Often a multiple-reed relay was used, with one of the reeds latching the relay, and the other or others performing logic or memory functions. Most reed relays in the crossbar switching systems of the 1940s through the 1970s were packaged in groups of five. Such a "reed pack" was able to store one decimal digit, encoded in a two-out-of-five code (74210 code variant) for easy validity checking by wire spring relay logic.