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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 and area code 210 in San Antonio. 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. In terms of geographic area covered, 312 is one of the smallest area codes in the nation, encompassing only a few square miles. ==History== Area code 312 was one of the original 86 number plan areas (NPAs) created in 1947 for the North American Numbering Plan. Given the rotary dial technology available at the time, AT&T kept the number of pulses to a minimum for the largest cities. Chicago was assigned a code with only six clicks, tied with 213 for the second-fastest that could be dialed under the original guidelines, after area code 212 assigned to New York City. The digits 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. Area code 312 originally served almost all of the Illinois side of the Chicago metropolitan area, except for portions of Grundy, McHenry, and Will counties in the area code for the rest of northern Illinois, 815. On November 11, 1989, the suburban portion split off as area code 708, with 312 reduced to Chicago. Within only five years, 312 was on the verge of exhaustion once again due to the proliferation of cell phones, pagers and fax machines. As a solution, area code 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 (along with New York City and Los Angeles). Since November 7, 2009, both 312 and 773 have been overlaid by 872. The first production #4ESS toll switch was installed in Chicago in January 1976.〔(Telephone World )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Area code 312」の詳細全文を読む スポンサード リンク
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