Overlay complex

The method has been in use since 1992, and has been the exclusive method of area code relief since 2007. From the 1947 inception of the North American Numbering Plan in to 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 (one exception was area code 606 in eastern Kentucky; the populous areas of Lexington and the Cincinnati suburbs in northern Kentucky received the new area code 859, with other locations, including the Eastern Kentucky Coalfield, retaining 606). It gained numbering capacity by acquiring the central office codes of the other areas, because those were delegated to a new area code in the process. The existing central office codes in the separated areas are maintained in the new numbering plan area and so that the 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.