Exchange names

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. Each central office 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 central offices, possibly hosted in the same 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 indicated visibly on a dial telephone.

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 two letters from the central office name with four or five digits, which was 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, such as in the example image for the telephone number LAkewood-2697 in service in Lakewood, NJ (USA).

Telephone exchange names were used in many countries, but were phased out for numeric systems by 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.

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 the letter of the telephone number to the digits on the telephone dial. 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, 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 Direct Distance Dialing. For example, under this system, a well-known number in New York City was listed as PEnnsylvania 6-5000.

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