Area code 312 is the telephone area code for the Chicago Loop and surrounding areas of the city of Chicago, Illinois. Prior to the split on November 11, 1989, forming 708 (for much of the suburbs), 312 area code originally served much of Greater Chicago. From November 11, 1989, to October 12, 1996, 312 served only the entire city of Chicago, 773 was assigned for the remainder of the city outside the downtown area on that day. It is roughly bounded by North Avenue on the north, Ashland Avenue on the west, and 31st Street on the south, with Lake Michigan to the east. By size, it is one of the smallest area codes in the nation, encompassing only a few square miles. Since November 7, 2009, both 312 and 773 are overlaid by 872.
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Area code 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.[1]
| Illinois area codes: 217, 224, 309, 312, 331, 618, 630, 708, 773, 779, 815, 847, 872 | ||
|---|---|---|
| North: 773 | ||
| West: 773 | area code 312 (overlaid by 872) | East: Lake Michigan |
| South: 773 | ||