Ten-digit dialing is a telephone dialing procedure in the countries and territories that are members of the North American Numbering Plan (NANP). It is the practice of including the area code of a telephone number when dialing to initiate a telephone call. When necessary, the ten-digit number may be prefixed with the trunk code 1, which is referred to as 1+10-digit dialing or national format. History The implementation and expansion of the North American Numbering Plan between 1947 and 1992 preserved a long-standing practice in the United States and Canada that callers should only need to dial the local seven-digit telephone number when placing a call within the caller's exchange area or within the home numbering plan area (NPA). In seven-digit dialing. callers dialed the three-digit central office code and the four-digit station number of the telephone subscriber to reach in the same numbering plan area. Dialing of an area code before the telephone number, referred to as ten-digit dialing, was only necessary for foreign NPA (FNPA) calls. Some communities with significant parts on both sides of an area code boundary implemented central office code protection, where possible, to ensure the same seven-digit local number was not assigned in two different area codes in the same city to assure seven-digit dialing in the entire community. Code exhaustion relief When the demand for telecommunication services in a numbering plan area threatened to exceed the capacity of telephone numbers of a single area code, the local public service agency and the Federal Communications Commission initiated relief measures to mitigate the exhaustion, and assure continued service with minimal customer impact in the affected areas. Area code split The established practice of relief from 1947 to 1992 was to divide the affected numbering plan area into suitable parts, most often two, and assigned a new area code to one of the areas, while maintaining the existing area code in the other, most commonly the denser-populated or geographically or historically more significant part. This freed the telephone numbers already issued in the new area for use in the existing, now smaller, numbering plan area. This practice preserved seven-digit dialing in all affected areas. Area code overlay By 1992, a new method of relief became available from the proliferation of electronic switching systems that were based on stored program control (SPC), which reduced the need for electromechanical extensions of the switching fabric. This made it possible