Area code 312

Area code 312 is the telephone area code for downtown Chicago, which includes the Chicago Loop and its immediate environs. The plan area is completely surrounded by area code 773, which serves the rest of Chicago, along with overlay area code 872. 312 is an enclave area code, similar to area code 213 in Los Angeles, area code 514 in Montreal and area code 210 in San Antonio. It is roughly bounded by North Avenue on the north, Western Avenue on the west, and 35th Street on the south, with Lake Michigan to the east. In terms of geographic area covered, 312 is one of the smallest area codes in the nation, encompassing only a few square miles.

Area code 312 was one of the original 86 number plan areas (NPAs) created in 1947 for the North American Numbering Plan. Due to Illinois' dense population, particularly in the north, it was split between four NPAs, given that each NPA could only accommodate approximately 500 central offices.

Area code 312 originally served almost all of the Illinois side of the Chicago metropolitan area, except for portions of Grundy, Kane, McHenry, and Will counties in the area code for the rest of northern Illinois, 815. On November 11, 1989, the suburbs were assigned area code 708 and 312 was reduced to Chicago.

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