The North American Numbering Plan (NANP) is the numbering plan used for the Public Switched Telephone Network in the United States and other territories. The NANP was introduced in 1947 by AT&T and Bell Laboratories. It conforms to the international standard for numbering plans. The NANP numbers are ten digits in length, and in the format NXX-NXX-XXXX. N can be any 2-9 digit and X can be any digit 0-9. The first three digits are called the numbering plan area (NPA) code. Hence the area code is born. The second three digits are called the central office code or prefix. And the final four digits are called the line number (http://www.allareacodes.com/area_code_faqs.htm)