Area code 312 encompasses the Chicago Loop and surrounding areas in Chicago, Illinois. It is roughly bounded by North Avenue on the north, Halsted Street on the west, Lake Michigan on the east, and Roosevelt Road on the south. By size, it is one of the smallest area codes in the nation encompassing only a few square miles.
312 was one of the original 86 area codes created in 1947, then called "Numbering Plan Areas." With the rotary dialing technology available at the time, care was given to keep the number of "clicks" to a minimum for large cities, hence Chicago being assigned a code with only six clicks.
The first production #4ESS toll switch was installed in January 1976. Telephone World
Exchange split
Until 1989, 312 included most of the Chicago metropolitan area; at that time the suburbs became area code 708. In 1996, most of the city split off into 773 after the northern suburbs and western suburbs split into area codes 847 and 630 respectively, while the south & near-west suburbs kept 708.
