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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. For this reason, Chicago was assigned a code with only six clicks, tied with 213 for the second-fastest could be dialed under the original NANPA guidelines after the 212 area code assigned to New York City. (0 and 1 were not allowed as the first digit, the second digit was either 0 or 1, and the third digit could not be the same as the second digit.)
It originally served almost all of the Chicago metropolitan area. On November 11, 1989, most of the Chicago suburbs were split off as area code 708, with 312 reduced to Chicago itself. 773 was assigned for the remainder of the city outside of the downtown area on October 11, 1996, making Chicago one of the few cities in the nation to be split between multiple area codes. Since November 7, 2009, both 312 and 773 have been overlaid by 872.
Cook County - Population 5,194,675