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Area codes 815 and 779 are telephone area codes in the North American Numbering Plan (NANP) for most of northern Illinois outside the Quad Cities areas. It also serves the Joliet area and some of Chicago’s western and northwestern suburbs. 815 was one of the four original Illinois area codes set up in 1947. Area code 779 was assigned to the same numbering plan area in 2009 to form an overlay.

When the American Telephone and Telegraph Company (AT&T) created the first nationwide telephone numbering plan for Operator Toll Dialing in 1947, the state of Illinois was divided into four numbering plan areas (NPAs). They were generally laid out as a region in and around Chicago, a northern band outside Chicago, and two bands in the central and southern parts of the state. 815 was assigned to the northern band outside Chicago as one of the 86 original North American area codes.

After September 1956,^[1] the 815 numbering plan area was divided roughly in half along a north-westerly to south-easterly running line. Area code 309 was created by merging the western part of 815, including the Illinois side of the Quad Cities, with the northern portion of area code 217. The new area code was one of the first area codes not serving an entire state, but having the digit 0 in the middle position of the code. It was the only new area code created in Illinois between 1947 and the 1989 creation of area code 708.^[2]

In preparation of an overlay plan with the new area code 779, all calls from the 815 area needed to be dialed with an area code beginning on February 17, 2007. A month later, area code 779 was activated.

• List of North American Numbering Plan area codes
• List of Illinois area codes

• NANPA area code map of Illinois
• Area Code Overlay Approved for 815 Area Code - general information from AT&T, the RBOC for the area code

40°11′44″N 89°31′00″W / 40.195659°N 89.516602°W / 40.195659; -89.516602