List of North American Numbering Plan area codes

The North American Numbering Plan (NANP) divides the territories of its member countries into Numbering Plan Areas (NPAs), each identified by a three-digit code commonly called area code.

The rules for numbering NPAs do not permit the digits 0 and 1 as the leading digit of an area code. 0 has been used traditionally for operator-assisted telephone calls, and the digit 1 was traditionally ignored in switching equipment as a leading digit, because it could not be distinguished reliably from intermittent loop disconnection when subscribers operated telephones. Later, 1 was assigned as a trunk prefix for long-distance calls; 011 indicates international calling, 10 prefixes are part of a feature group to select alternate interexchange carriers on a per-call basis, and 11 is a vertical service code prefix. NPAs also cannot currently have 9 as a second digit. This is to allow for a future format expansion to 4 digits or more. The expansion scheme would involve inserting a 9 as a second digit in all existing NPAs—450 would become 4950—and telephone switching equipment would recognize the 9 as a number being dialed in the new expanded format, otherwise it would be considered dialed as a legacy three-digit NPA. This would allow a transition period for dialing in both formats. After the transition period, digits other than 9 would be allowed as a second digit. Furthermore, the central office prefix, i.e. the leading three digits of the seven-digit subscriber number, cannot begin with 0 and 1. This distinguished NPAs from central office codes. "Regular" area codes also cannot have the last two digits match (such as 322 or 755); these are considered "easily recognizable codes" (ERC). Examples of these are toll-free codes 888, 877, 866, 855, and 844.