A telephone exchange name or central office name was a distinguishing and memorable name assigned to a central office. It identified the switching system to which a telephone was connected, and facilitated the connection of telephone calls between switching systems in different localities.
While small towns and rural areas might each be served by a single exchange, large cities were served by multiple switching systems, either distributed in the community constituting multiple exchange areas, or sometimes hosted in the same building to serve a densely populated area. Central offices were usually identified by names that were locally significant. 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 the area was unique. These letters were mapped to the digits of the dial, which was indicated visibly on the dial's numbering plate.
Several systematic telephone numbering plans existed in various communities, typically evolving over time as the subscriber base outgrew older numbering schemes. A widely used numbering plan was a system of using one or two letters from the central office name with four or five digits. Such systems were designated as 2L-4N or 2L-5N, or simply 2–4 and 2–5, respectively, 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 each with the digits 2 through 9 on the dial.
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 to be dialed were capitalized, while the rest of the name was shown in lower case.
Telephone exchange names were used in many countries, but were phased out in favor of numeric systems in the 1960s. In the United States, the demand for telephone service outpaced the scalability of the alphanumeric system and after introduction of area codes for direct-distance dialing, all-number calling became necessary. Similar developments followed around the world, such as the British all-figure dialling.
Background
In the early, small telephone networks, it was customary to initiate a connection to another subscriber by requesting the name of the desired party from the operator. While this method persisted into the 1920s in very small communities,A.E. Van Hagan (October 1927) Bell Telephone Quarterly Volume 6, No. 4, p.231. Selection of Central Office Names growth of the business soon made this impractical, and subscriber stations were assigned telephone numbers. Single exchanges (central offices) were typically named after the municipality or location, so that calls to another town could be easily identified. Cities soon needed additional branch offices some distance from Central, to accommodate the subscriber base and expanding area, as a single office typically served a maximum of ten thousand telephone numbers. Often, additional central offices might be named by the directions of the compass, North, South, East, and West. But many cities chose other naming schemes, using locally significant names of districts, parks, or other well-known features, such as Market. A caller would request a connection to Market 1234, for example. The selection of central office names was conducted in a careful manner to avoid misunderstanding of the verbal requests.
For automatic telephone service, impulse senders (dials) were installed on customer telephones, so that subscribers did not need operators to initiate a call, but simply dial the directory number themselves. This required a digit or letter identification of central offices, so that the central office for the recipient could be dialed before the line number. Telephone dials were typically supplemented with letters next to the numerals on the dial, as seen in the accompanying photo, so that a name could be dialed by its first letter, or by multiple letters.
United States and Canada
In the United States, the most-populous cities, such as New York City, Philadelphia, Boston, and Chicago, initially implemented dial service with telephone numbers consisting of three letters and four digits (3L-4N) according to a system developed by W. G. Blauvelt of AT&T in 1917. This system mapped letters of the alphabet to digits on the telephone dial. In 1930, New York City converted to a 2L-5N telephone numbering plan. Most other major Canadian and US cities, such as Toronto and Atlanta, were converted from manual exchanges using four digits to a 2L-4N numbering plan. For example, in Montréal, ATwater 1234 was dialed as six pulls on the dial (AT1234) to send the digit sequence 281234. Eventually, starting in the late 1940s, all local numbering plans were changed to the 2L-5N system to prepare for nationwide Operator Toll Dialing. For example, under this system, a well-known number in New York City was listed as PEnnsylvania 6-5000.
In small towns with a single central office, local calls typically required dialing only four digits at most. A toll call required the assistance of an operator, who asked for the name of the town and the station number. Some independent telephone companies, not part of the Bell System, also 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. When the technology first appeared in the Mulberry exchange, subscriber telephones had no dials, and placing a call required no change for subscribers—they asked an operator to ring their called party as usual. The operator keyed the number into the panel equipment, instead of making cord connections manually. The panel 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.
Several representations of telephone numbers using central office names capitalized and emboldened the leading letters that were dialed, for example: Kenmore 9392 is a five-pull (1L-4N) small-city telephone number for the Kenmore exchange in Fort Wayne, Indiana. MArket 7032 is a six-digit (2L-4N) telephone number. This format was in use from the 1920s through the 1950s, and was phased out c. 1960. BALdwin 6828 is an urban 3L-4N example, used only in the largest cities before conversion to two-letter central office names. 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, but not emboldened, as the first letter of the second word.
In print, such as on business cards or in advertisements, the central office name was often shown only by two letters: TEmpleton 1-6400 would often appear as TE 1-6400.
If the central office was known by a name, but no letters were dialed, it was common to capitalize only the first letter of the central office without emboldening the first letter, 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. Often these were geographically significant names, such as the town's name.
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 an automatic switching system, 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 6 operator who would in turn dial the JUniper 4 number.
In the 1940s, the Bell System developed the nationwide telephone number plan for Operator Toll Dialing, a system of initially eighty-six numbering plan areas (NPAs) that were assigned the first set of area codes. These were used at first only by switchboard operators to route trunk calls between numbering plan areas, but were the foundation for the North American Numbering Plan, The 2L-5N system for the local directory number became the North American standard Direct long-distance dialing by customers, using the three-digit area code and a seven-digit telephone number, commenced in the 1950s.
During the 1950s, cities still using five or six-digit numbers converted to the new method of 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 may have been combined into a new 2L-5N exchange area. 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.
When mobile radio telephone service was offered, some telephone companies used letters based on various prefixes for unit identifiers (e.g. JL6-1212), or to identify radio channels (e.g. channel JR).
Standardization +U.S. Mapping of letters to dialed digits in the 1950sdialed digitletters1 2 A B C3 D E F4 G H I5 J K L6 M N O7 P R S8 T U V9 W X Y0 Z In the early 1950s, AT&T established a list of recommended exchange names that were the result of studies to minimize misunderstandings when spoken. The recommendation was intended for newly established exchanges, and did not mandate the renaming of existing historical names. The number sequences 55, 57x, 95x, and 97x had no exchange names specified, as the mappings for the digits 5, 7, and 9 had no vowels, thus making it difficult to find names with those consonant combinations. As a result, those numbers were very seldom assigned to exchanges. However, KLondike was used for 55x in San Francisco and Columbus, Ohio, and WRigley 5 (975) in Chicago (Wrigley Field). On the telephone dial, letters were mapped to digits using the assignments shown in the table (right).
The following is AT&T's recommended list of central office names as published in 1955, sorted by the first two digits of the three-digit central office code.