In telecommunications, an overlay plan is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA). Overlaying numbering plan area codes is practiced in territories of the North American Numbering Plan (NANP) to prevent exhaustion of telephone numbers and central office codes in growth areas. History From the inception of the North American Numbering Plan in 1947 until 1992, the only method of introducing new area codes was the area code split. It divided an existing numbering plan area (NPA) into multiple regions, one of which retained the existing area code. The other parts were assigned a new NPA code each. The retaining area was usually the historically more established or developed place, and required no numbering changes. It gained numbering capacity by acquiring the central office prefixes of all other areas. The existing central office codes in the separated areas are maintained in the new numbering plan area, so that the only change for existing subscribers is the change of area code. Thus, only area code references, where present for long-distance calling, required updating in directories, letterheads and business cards. With the proliferation of electronic central office switching during the 1980s, it became possible to implement another method for expanding numbering plan resources, the area code overlay. In an overlay numbering plan, an additional area code is assigned to an existing numbering plan area, thereby avoiding the need for existing customers to change telephone numbers. The first use of that solution was in New York City in 1992, when area code 917 was overlaid to two numbering plan areas, Manhattan's area code 212, and the outer boroughs' area code 718. Methodology The overlay method required ten-digit dialing, by including the area code, for all subscribers for all calling destinations, local or long-distance. Exceptions Near numbering plan area boundaries, special situations arose at times when established communities extended on both sides of the boundary. To preserve local community integrity, identity, and convenience, a party could dial an office prefix for a local call with only seven-digits, but the destination was technically located in the adjacent number plan area. If a subscriber called a distant office prefix with the same area code, the call would require the dialing prefix 1 before the seven- or ten-digit number. A similar practice was implemented on a large scale in the metropolitan area of Washington, DC, and its suburbs