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In the original North American Numbering Plan of 1947, all of the Chicago area was covered by area code 312, the rest of northern Illinois was 815, central Illinois was 217, and southern Illinois was 618. In 1966, area code 309 was split off to cover western central Illinois (which was previously the southwestern part of area code 815). In 1990, area code 708 was split off to cover all of the suburbs in the Chicago metropolitan area (which is now covered by 847/224, 630/331, and 708) while the city of Chicago kept the original 312. Area codes 847 (north suburbs) and 630 (west suburbs) were split off from the suburban area code 708 in 1996. Shortly after in 1996, area code 773 was created as an area code split to cover the residential parts of the city of Chicago while downtown kept the original 312 area code. Area code 847 almost immediately exhausted its numbers, so an overlay area code, 224, was implemented in 1996 for supplemental numbers. However, mandatory 11-digit dialing was not in effect until 2002. In March 2007, the overlay area code 331 was added for 630 area, as was the overlay area code 779 for 815. With the depletion of new numbers in area codes 312 and 773, an overlay of both of them, area code 872, was created in November 2009, beginning 11-digit dialing within the city limits of Chicago. 
872