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, 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:
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