Editing Overlay complex (section)

==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 each assigned a new NPA code. 

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 code]]s of the other areas, because those were delegated to a new area code in the process.<ref>{{Cite web|url=https://nationalnanpa.com/number_resource_info/co_codes.html|title=NANPA : Numbering Resources - Central Office Codes|website=nationalnanpa.com|access-date=2019-10-10}}</ref> 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. Local, seven-digit dialing was unaffected by the area code change.

With the proliferation of [[electronic switching system]]s starting in the 1980s, it became possible to implement another method for expanding numbering plan resources, the area code ''overlay''. In an overlay numbering plan, also called ''overlay complex'', an additional area code is assigned to an existing numbering plan area, thereby avoiding the need for existing customers in the reassigned regions 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 [[area code 718]] in the outer boroughs.