Methodology
Prior to the introduction of overlay plans, the method of introducing new area codes (also known as numbering plan areas or NPAs) in a region was to divide the existing territory of one area code into two (or more) pieces, allowing the more established or developed section to retain the original area code, and changing the numbering space of the other section(s) to have a new area code. For example, the original area code for the entire state of Washington was 206; today 206 applies to only the city of Seattle and immediate vicinity. This practice became known as a split plan.
In a number of cases, overlay plans were implemented on a special case basis to implement specialized dialing plans. In some areas, a party in one area code could dial a prefix which was local, but in a different area code, with only 7 digits. If they were calling a distant prefix in the same area code, they would either have to dial 1+ the number or 1+area code+ the number.
This practice was done on a large scale in Washington, DC and its suburbs in Maryland and Virginia. Until 1991, any telephone in the Washington, DC metropolitan area could dial any local number in the District, Maryland or Virginia by dialing the 7-digit number. This setup was possible because the entire Washington metropolitan area is a single LATA. The way this was implemented was that every telephone in Maryland and Virginia that was in the Washington Metropolitan area was given a 'hidden' phone number consisting of the same number in the District's area code 202, essentially making area code 202 an overlay over the entire region. This meant a Virginia number such as 703-931-xxxx could also have been dialed as 202-931-xxxx, while a Maryland number such as 301-585-xxxx could also be dialed as 202-585-xxxx. It was thus possible for telephone customers in the Washington metro area to dial a number within the metro with only seven digits. However, this also meant no prefix could be duplicated in any of the three territories. By 1991 the demand for more numbers was too great, the use of 202 as a de facto overlay was discontinued, and callers in the area dialing an out-of-area-code number had to dial the full 10 digit number.
Read more about this topic: Overlay Plan
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