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A telephone exchange name or central office name was a distinguishing and memorable name assigned to a telephone exchange. It identified the central office to which a telephone was connected. Each exchange served a maximum of 10,000 subscriber lines identified by the last four digits of the telephone number. Areas or cities with more subscribers were served by multiple exchanges, possibly hosted in the same central office building. The leading letters of a central office name were used as the leading components of the telephone number representation, so that each telephone number in an area was unique. These letters were mapped to digits which was usually indicated visibly on a dial telephone. Several systematic telephone numbering plans existed in various communities, sometimes evolving over time as the subscriber base outgrew older numbering schemes. A widely used numbering plan was a system of using two letters from the central office name with five digits, which was designated as ''2L-5N'', but some large cities initially selected plans with three letters (''3L-4N''). In 1917, W. G. Blauvelt of AT&T proposed a mapping system that displayed three letters with the digits 2 through 9 on the dial.〔''A History of Engineering and Science in the Bell System: The Early Years (1875-1925)'' p578 by M. D. Fagen (editor) & Bell Labs technical staff (1975, Bell Telephone Laboratories)〕 Telephone directories or other telephone number displays, such as in advertising, typically listed the telephone number showing the significant letters of the central office name in bold capital letters, followed by the digits that identified the subscriber line. On the number card of the telephone instrument, the name was typically shown in full, but only the significant letters were capitalized, while the rest of the name was shown in lower case, such as TAylor 4725, or as small caps. Telephone exchanges were used in many countries, but were phased out for all-numeric system by the 1960s. In the United States, the introduction of area codes, direct-distance dialing were precursors to all-number calling systems around the world, such as the British all-figure dialling. ==United States and Canada== In the United States, the most-populous cities, such as New York, Philadelphia, Boston, and Chicago used telephone numbers consisting of three letters and four digits (''3L-4N''). Most other major Canadian and US cities, such as Toronto and Atlanta, were converted from manual exchanges using four digits to a local 2L-4N numbering plan. For example, ''ATwater 1234'' was dialed as six digits, ''AT-1234'' or ''28-1234'', in Montréal. Eventually, all local numbering plans were changed to the 2L-5N system to prepare for Direct Distance Dialing. For example, under this system, a number in New York City was shown as PEnnsylvania 6-5000. In small towns with a single central office, local calls typically required dialing four or five-digits at most, without using named exchanges. A toll call required the assistance of an operator, who asked for the name of the town and the local number. Some independent telephone companies, not part of the Bell System, did not implement central office names. In 1915, newly developed panel switching systems were tested in the Mulberry and Waverly exchanges in Newark, New Jersey. This type of switch was later, from the early 1920s through the 1930s, installed in large metropolitan areas in the Bell System. By the 1950s twenty cities were served by this type of office. When the technology first appeared, it was transparent to subscribers — the customer asked an operator to ring their called party as usual. However, the operator keyed the number into the panel equipment, instead of making cord connections manually.〔http://www.telephonetribute.com/switches_survey_chapter_5.html〕 From the time of these first conversions to automated equipment in the 1920s, through the conversions of most manual equipment by the 1960s, it was necessary for telephone numbers to be represented uniformly across the nation. By Bell System policy, customers never needed to be concerned about whether they were calling an automatic or a manual exchange. The 2L-5N system became the North American standard, as customer-dialed long distance service came into use in the 1950s. Several standard formats of telephone numbers, based on central office names, capitalized the leading letters that were dialed, for example: *''BALdwin 6828'' is a typical urban North American 3L-4N example, used in only large cities before conversion to two-letter central office names. *''MArket 7032'' is a typical North American six-digit (2L-4N) telephone number. This format was in use from the 1920s through the 1950s, and was phased out by ca. 1960. *''ENglewood 3-1234'' is an example of the 2L-5N format, gradually implemented continent-wide starting in the 1940s, in preparation for DDD. *''MUrray Hill 5-9975'' is another example of the 2L-5N format, one of the Ricardos' numbers on ''I Love Lucy''. The H in Hill, although not dialed, is still capitalized as the first letter of the second word. In print, such as on business cards or in advertisements, the full central office name was often shown only by the two letters: *TEmpleton 1-6400 would appear as TE 1-6400. If the letters were not dialed, it was common to capitalize only the first letter of the central office, e.g., ''Main 600W'' or ''Fairmont 33''. Such numbers were assigned typically in manual offices, and the name would be spoken by a subscriber when requesting a destination. In large cities with coexisting manual and dial areas, the numbering was generally standardized to one format. For example, when the last manual exchange in San Francisco was converted to dial in 1953, the numbers had for several years been in the format of JUniper 6-5833. JUniper 4 was automatic, but JUniper 6 was manual. To call JUniper 6 from JUniper 4, the subscriber dialed the number and it was displayed to the B-board operator at JUniper 6, and that operator would complete the connection manually. In the other direction, to call JUniper 4 from JUniper 6, the subscriber would lift the receiver and speak to the JUniper 4 operator, who dial the JUniper 6 number.〔http://doc.telephonecollectors.info/dm/53_PTM_San_Franciscos_Last_Manual_Exchange.pdf〕 During the 1950s, cities using six-digit numbers converted to seven-digit dialing. Typically, several six-digit (2L-4N) exchanges were co-located in one building already, with new ones added as old ones had filled up. After the conversion, they were combined into a new 2L-5N exchange. For example, the CHerry, FIllmore, ATwater, and KLondike exchanges might be converted to OXford 1, 3, 6, and 7. Usually customers would keep the same station numbers. In the 1940s, the Bell System developed the North American Numbering Plan, a system of initially 86 area codes which were used initially only by switchboard operators to route trunk calls between plan areas. Direct long-distance dialing by customers using the 3-digit area code and a 7-digit telephone number, commenced in the 1950s. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Telephone exchange names」の詳細全文を読む スポンサード リンク
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