| 7-digit dialing ： ウィキペディア英語版|
Seven-digit dialing is a telephone dialing procedure customary in the territories of the North American Numbering Plan, for dialing telephone numbers in the local calling area. It is also sometimes known as ''local format'' or ''network format''.
Originally, telephone exchanges consisted of manual boards operated by switchboard operators. Numbers were typically four digits or fewer for local calls within an exchange due to practical limitations (if each line had a jack on the switchboard, four digits or 10000 possible numbers filled a 100 x 100 board). As the number of subscribers grew, multiple exchanges served individual neighborhoods of large cities. A city telephone number consisted of an exchange name and four digits, such as "Pennsylvania 5000". A rural telephone number, often party line, was often up to four digits plus a letter or letter and digits to indicate which of the multiple parties on the line was desired.
Various schemes were used to convert these to dialable numbers as dial 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). A few of the largest cities, such as New York, used seven dial pulls ("PENnsylvania 5000" became PEN-5000 and later PEnnsylvania 6-5000, dialled PE6-5000 or 736-5000).
The initial 86 area codes were assigned in 1947 as routing codes for operator calls; the first cross-country Bell System direct distance dial call was made in 1951. 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 1950's, 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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