Editing Seven-digit dialing (section)

==History==
Originally, [[telephone exchange]]s consisted of manual switchboards operated by [[switchboard operator]]s. Telephone numbers had typically two to four digits, depending on the size of the community. As the number of subscribers grew, multiple exchanges served individual neighborhoods of large cities. Multiple exchanges were identified by a central office name and typically four digits, such as "Pennsylvania 5000". A rural telephone number, often [[party line (telephony)|party line]], had often up to four digits and a letter or letter and digits to indicate which of the multiple parties on the line was desired.

Various methods were used to convert these to dialable numbers as dial systems replaced manual switchboards; many moderately-large cities used a 2L-4N format where "ADelaide 1234" would be dialled as AD-1234 (23-1234, a six-digit local call). The four largest cities ([[New York City|New York]], [[Chicago]], [[Philadelphia]], and [[Boston]]) used seven digits. In New York, for example, "PENnsylvania 5000" became PEN-5000 and later [[PEnnsylvania 6-5000]], dialled PE6-5000 or 736–5000). New York used the 3L-4N format from 1920, when dial telephones were first introduced there, until 1930, when it switched to 2L-5N.

The [[original North American area codes]] were assigned in 1947 as routing codes for operator calls, but by 1951, the first cross-country [[Bell System]] [[direct distance dial]] was placed directly from a subscriber station. The system was based on fixed-length numbers; a direct-dial long-distance call consisted of a three-digit area code and a seven-digit local number. Numbers in 2L-4N cities (such as [[Montréal]] and [[Toronto]]) were systematically lengthened to seven digits in the 1950s, a few exchanges at a time, so that all local numbers were seven digits when direct distance dialling finally came to town.

Exchange prefixes were added to small-town numbers to extend four or five-digit local numbers to the standardised seven-digit length, matching in length the then-longest local numbers in the largest major US markets.