Editing Seven-digit dialing (section)

==Structure==
Within the multinational calling area administered by the North American Numbering Plan, telephone numbers are composed of three fixed-length fields: a three-digit numbering plan area (NPA) code ([[area code]]), a three-digit (NXX) central office code, and a four-digit (XXXX) station number.

In seven-digit dialing, only the central office code and the station number is dialed, indicating that the call destination is within the local area code. This was the standard in most of North America from the 1950s onward. In some small villages with only one local exchange, it may have been permissible to dial only the four-digit station number. Even after exchange names were introduced, it was possible in many small cities to call local numbers by dialing only the five digits which followed.

A long-distance call within the same area code could often be dialed as 1+7D, without using an area code. The scheme relied on the second digit of an area code being 0–1 and the second digit of a local exchange being 2–9. This dialing plan was incompatible with the introduction of [[area code 334]] and [[area code 360]], and was therefore eliminated by January 1, 1995, in the United States, and by September 1994 in Canada. It was also eliminated as early as 1981 in some numbering plan areas in the United States that had introduced interchangeable central office codes.

Interchangeable central office codes are central office codes (NXXs) which, with a zero or one as the middle digit, resemble and duplicate area codes in the pre-1995 format. They were introduced to postpone area code splits in major cities such as New York City and Los Angeles, but in 1988, AT&T/Bellcore made them mandatory for area codes nearing exhaust of non-interchangeable codes. The Massachusetts 617/508 split was the last one before the policy changed—617 did not yet have interchangeable NXXs at the time. Area codes with interchangeable NXXs had mandatory 10-digit long-distance dialing in order to allow exchanges to distinguish between intra- and inter-area code calls. From 1988 to 1994, few area codes splits were possible due to the dwindling supply of area codes, so conservation measures became necessary.

As of 1995, with the introduction of [[interchangeable NPA code]]s, nearly all code combinations are useful as NPAs or as NXXes.