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The North American Numbering Plan (NANP) divides the territories of its members into geographic numbering plan areas (NPAs). Each NPA is identified by one or more numbering plan area codes (NPA codes, or area codes), consisting of three digits that are prefixed to each local telephone number having seven digits. Numbering plan areas may be assigned multiple area codes to provide adequate numbering resources, in which case the local numbering plan is called an overlay. Area codes are also assigned for non-geographic purposes. The rules for numbering NPAs do not permit the digits 0 and 1 as the leading digit, which also applies for central office codes. NPAs with 9 as the second digit are reserved for future format expansion. Area codes with two identical trailing digits are considered easily recognizable codes (ERC), which are reserved, or used for systemwide services.

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