Chicago was given area code 312 in 1947 to minimize the rotary "click count," requiring the second-lowest six clicks on a rotary dial.

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It was among the first 86 area codes created in 1947.

From Wikipedia:

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.)