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. Both area codes are overlaid with 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, Kankakee, 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.
The 1989 split was intended as a long-term solution to relieve exchanges in one of the largest toll-free calling zones in the nation. However, within only five years, 312 was on the verge of exhaustion once again due to the proliferation of cell phones, pagers and fax machines. Additionally, the Chicago LATA extends for some distance into northwest Indiana (Gary, Hammond, East Chicago), and several numbers in northern Indiana's 219 were not available for use. As a solution, all of the city outside of the downtown area was split off as area code 773 on October 11, 1996, making Chicago one of the few cities in the nation to be split between multiple area codes (along with New York City and Los Angeles). Since November 7, 2009, both 312 and 773 have been overlaid by 872. With the implementation of 872, local calls in Chicago must be dialed with the full 11-digit number (1 plus the area code plus the number).
|Illinois area codes: 217, 224/847, 309, 312, 331/630, 618, 708, 773, 779/815, 872|
|West: 773||area code 312 (overlaid by 872)||East: Lake Michigan|