Original North American area codes

The original North American area codes were established by the American Telephone and Telegraph Company (AT&T) in 1947, after the demonstration of regional Operator Toll Dialing during the World War II period. The program had the goal of speeding the connecting times for long-distance calling by eliminating intermediary telephone operators. Expanding this technology for national use required a comprehensive and universal, continent-wide telephone numbering plan.

The new numbering plan established a uniform destination addressing and call routing system for all telephone networks in North America which had become an essential public service.^[1] It had the eventual benefit of direct distance dialing (DDD) by telephone subscribers, a feature by which a caller may, without operator assistance, call any other user outside the local calling area (typically requiring extra digits to be dialed).

The initial Nationwide Numbering Plan of 1947 established eighty-six numbering plan areas (NPAs) that conformed mainly to existing U.S. state and Canadian provincial boundaries, though fifteen states and provinces were subdivided further. Forty NPAs were mapped to entire states or provinces. Each NPA was identified by a three-digit area code used as a prefix to each local telephone number. The United States received seventy-seven area codes, and Canada nine. The initial system of numbering plan areas and area codes was expanded rapidly during the ensuing decades, and established the North American Numbering Plan (NANP).

History

Early in the 20th century, the American and Canadian telephone industry had established criteria and circuits for sending telephone calls across the vast number of local telephone networks on the continent to permit users to call others in many remote places in both countries. By 1930, this resulted in the establishment of the General Toll Switching Plan, a systematic method, with technical specifications, for routing calls between two major classes of routing centers, Regional Centers and Primary Outlets,^[2] as well as thousands of minor interchange points and tributaries. Calls were forwarded manually between stations by long-distance operators who used the method of ringdown to command remote operators to accept calls on behalf of customers. This required long call set-up times with several intermediate operators involved. When making a call, the originating party would typically have to hang up and be called back by an operator once the call was established.

The introduction of the first Western Electric No. 4 Crossbar Switching System in Philadelphia to commercial service, in August 1943,^[3] automated the process of forwarding telephone calls between regional switching stations. For the Bell System this was a beginning for the ability of their long-distance operators to dial calls directly to distant telephones.^[4] While automatic switching decreased the connection times from as much as fifteen minutes to approximately two minutes for calls between far-away locations, each intermediate operator still had to determine special routing codes unique to their location for each call. To make a nationwide dialing network an efficient, practical reality, a uniform nationwide numbering plan was needed so that each telephone on the continent had a unique address that could be used independently from where a call originated. Such a system is called destination routing.

With this goal, AT&T developed a new framework during the early 1940s, termed Operator Toll Dialing, which was begun by the installation of a newly developed toll switching system in Philadelphia in 1943. In 1945, the American Telephone and Telegraph Company declared this effort a major post-war project for the Bell System,^[5] and proceeded with periodical communications to the general telecommunication industry via the Dial Interexchange Committee of the United States Independent Telephone Association (USITA),^[6] which disseminated the project's progress to its members via industry journals and conference contributions. The planning transitioned to implementation, when Ralph Mabbs presented the results in a talk at the Fiftieth Anniversary Meeting of the Independent Telephone Association, on October 14, 1947.^[7]

A fundamental requirement for the success of automated toll dialing was a new telephone numbering plan, which became known as the Nationwide Numbering Plan. This numbering plan accounted initially for seventy-seven area codes in the United States and nine in Canada. With the construction of technical infrastructure for automated toll dialing, the allocations needed to be changed in many states, adding numerous additional area codes during the next decade. By 1975, the numbering plan was known as the North American Numbering Plan, as efforts were in progress to expand the system beyond the United States and Canada.

Numbering plan requirements

Building a nationwide network in which any telephone could be dialed directly required a systematic numbering system that was easy to understand and communicate. Local telephone numbers varied greatly across the country, from two or three digits in small communities, to seven in the large cities.

By the time the Bell Laboratory engineers began efforts to involve the broader industry bodies in 1945, definite concepts had been developed for Operator Toll Dialing. A crucial requirement was the conversion of all participating telephone networks to a universal numbering plan. In 1947, Ralph Mabbs recalled the specifications for this numbering plan as follows:^[7]

• A distinctive telephone number for each telephone in the United States and Canada.
• The minimum number of digits which will provide for growth and new services.
• Minimum changes in customers' numbers.
• Minimum changes in local dialing practice.
• Least cost for equipment changes.
• Minimum reference by operators to bulletins and route guides to gain speed of service advantages.
• Provisions for operators to directly reach other operators at distant toll centers.

Telephone dial number card of c.1948 with the local telephone number 4-5876 in Atlantic City, NJ, using the central office prefix 4, later converted to AT4

Face of a 1939 rotary telephone dial with the telephone number LA-2697, which includes the first two letters of Lakewood, New Jersey, as the central office prefix, later converted to LA6.

Based on the precedent and experience with the large-city dial systems in the nation, the designers decided to direct all telephone companies in the nation to standardize the local telephone networks to seven-digit local telephone numbers before they could participate with Operator Toll Dialing. This required few or no changes in the nation's largest cities, but in the smaller communities the shorter telephone numbers had to be padded with additional digits in a transparent, easily understandable manner, so that extra digits were not always needed when dialing other local subscribers. By 1955, AT&T disseminated a formal publication of network documentation, specifications, and recommendations to the telephone industry, entitled Notes on Nationwide Dialing.^[7][8][9]

Central office prefixes

Most automatic dial switching systems were designed since the early 1920s to provide service for as many as ten-thousand subscriber lines. Each of these switching systems constituted a local telephone exchange, formally known as a central office. Therefore, each telephone connected to a central office had a unique four-digit line or station number.

In larger communities that required multiple central offices to account for the service need of their population, extra digits were added to the telephone number, preceding the line number. Such extra digits were dialed when calling a telephone connected to another switching system in the same city or in a nearby community, and served as routing codes to those central offices. Central office prefixes had already been used in the cities' dial systems since the 1920s, and were typically dialed by subscribers as the initial letters of the exchange name, but only the largest of cities used three digits or letters. This practice and familiarity was preserved in the initial formulation of the new numbering plan, but was standardized to a format of using two letters and one digit in the prefix, resulting in the format 2L–5N (two letters and five numerals) for the subscriber telephone number.^[9]

For most cities, this conversion required the addition of extra digits or letters to the existing central office prefix. For example, the Atlantic City, New Jersey, telephone number 4-5876 was converted to AT4-5876 during the 1950s. Complete replacement of existing prefixes was necessary in the case of conflicts with another office in the state. Duplication of central office names, or an identical mapping of two different names to digits, was not uncommon. In practice, the conversion of the nation to this numbering plan took decades to accomplish and was not complete before the alphanumeric number format was abandoned during the 1960s in favor of all-number calling (ANC).

In addition to the central offices that provided the subscriber lines for each telephone (wire centers), the toll routing system included special switching facilities that routed long-distance calls between end offices. Each of these toll offices also received an assignment of a unique three-digit toll office code. To reach another operator in another central office or toll office, an operator dialed only the office code of the destination.

Numbering plan areas

By 1945, initial concepts for a nationwide numbering plan anticipated a division of the continent into between fifty and seventy-five numbering plan areas.^[10] For this size of the network, a unique two-digit code for each numbering plan area (NPA) would have been sufficient. However, AT&T wanted to preserve existing dialing practices by only dialing the local number for local calls; it was therefore necessary to distinguish the NPA codes from central office codes automatically by the switching system. Central office codes already could not have the digits 0 and 1 in either of the first two positions, because no letters were mapped to those to represent exchange names. This was the opportunity for distinction, but only when 0 or 1 were used in the second position, because switching systems already suppressed single loop interruption (corresponding to 1) automatically, and 0 was used to reach an operator or long-distance desk.^[9]

Therefore, numbering plan area codes, often termed just area codes, were defined to have three digits, with the middle digit being 0 or 1. Area codes with the middle digit 0 were assigned to numbering plan areas that comprised an entire state or province, while jurisdictions with multiple numbering plan areas received area codes having 1 as the second digit. This pattern lasted only until 1956, when New Jersey's "201" was divided.

The geographic layout of numbering plan areas across the North American continent was chosen primarily according to national, state, and territorial boundaries in Canada and the United States.^[7] While originally considered, no numbering plan area in the United States included more than a single state, but in Canada NPA 902 comprised all three provinces of The Maritimes in the far east. The largest states, and some states with suitable call routing infrastructure were divided into smaller entities, resulting in fifteen states and provinces that were subdivided further, creating 46 NPAs. Forty NPAs were assigned to entire states or provinces.

1947 division of the North American continent into 86 numbering plan areas (boundaries approximate)[7]

The original configuration of the North American Numbering Plan assigned eighty-six area codes in October 1947, one each to every numbering plan area.

The territories of the United States, which included Alaska, and Hawaii, did not receive area codes at first, nor did the territories of Canada or Newfoundland and Labrador, which was a British dominion at the time.^[7]

Area code	Country/State ID	State, province, or region
201	US:NJ	New Jersey
202	US:DC	District of Columbia
203	US:CT	Connecticut
204	CA:MB	Manitoba
205	US:AL	Alabama
206	US:WA	Washington
207	US:ME	Maine
208	US:ID	Idaho
212	US:NY	New York (New York City)
213	US:CA	California (southern, including Los Angeles)
214	US:TX	Texas (northeastern, including Dallas/Fort Worth)
215	US:PA	Pennsylvania (southeastern, including Philadelphia)
216	US:OH	Ohio (northeastern, including Cleveland)
217	US:IL	Illinois (central)
218	US:MN	Minnesota (except southeastern part of state)
301	US:MD	Maryland
302	US:DE	Delaware
303	US:CO	Colorado
304	US:WV	West Virginia
305	US:FL	Florida
306	CA:SK	Saskatchewan
307	US:WY	Wyoming
312	US:IL	Illinois (Chicago metropolitan area)
313	US:MI	Michigan (southeast, including Detroit)
314	US:MO	Missouri (eastern, including St. Louis)
315	US:NY	New York (central upstate, including Syracuse)
316	US:KS	Kansas (southern half, including Wichita)
317	US:IN	Indiana (northern two-thirds, including Indianapolis)
319	US:IA	Iowa (eastern third)
401	US:RI	Rhode Island
402	US:NE	Nebraska
403	CA:AB	Alberta
404	US:GA	Georgia
405	US:OK	Oklahoma
406	US:MT	Montana
412	US:PA	Pennsylvania (western, including Pittsburgh)
413	US:MA	Massachusetts (western, including Springfield)
414	US:WI	Wisconsin (southern and northeastern, including Milwaukee)
415	US:CA	California (northern/central, including San Francisco and Oakland)
416	CA:ON	Ontario (southern part, including Toronto, below a westward line from Oshawa)
418	CA:QC	Quebec (eastern half, including Québec City)
419	US:OH	Ohio (northwest, including Toledo)
501	US:AR	Arkansas
502	US:KY	Kentucky
503	US:OR	Oregon
504	US:LA	Louisiana
505	US:NM	New Mexico
512	US:TX	Texas (central and southern, including Austin and San Antonio)
513	US:OH	Ohio (southwest, including Cincinnati)
514	CA:QC	Quebec (western half, including Montreal)
515	US:IA	Iowa (central, including Des Moines)
517	US:MI	Michigan (south-central portion of Lower Peninsula, including Lansing)
518	US:NY	New York (northeastern, including Albany)
601	US:MS	Mississippi
602	US:AZ	Arizona
603	US:NH	New Hampshire
604	CA:BC	British Columbia
605	US:SD	South Dakota
612	US:MN	Minnesota (southeastern portion, including Minneapolis)
613	CA:ON	Ontario (northern part resulting from a westward dividing line from Oshawa on Lake Ontario)
614	US:OH	Ohio (southeast, including Columbus)
616	US:MI	Michigan (Grand Rapids, Upper Peninsula, western portion of Lower Peninsula)
617	US:MA	Massachusetts (eastern, including Boston)
618	US:IL	Illinois (southern, including East St. Louis and Carbondale)
701	US:ND	North Dakota
702	US:NV	Nevada
703	US:VA	Virginia
704	US:NC	North Carolina
712	US:IA	Iowa (western third, including Sioux City)
713	US:TX	Texas (southeastern, including Houston)
715	US:WI	Wisconsin (northern)
716	US:NY	New York (western, including Buffalo and Rochester)
717	US:PA	Pennsylvania (eastern half, except for the Delaware and Lehigh valleys)
801	US:UT	Utah
802	US:VT	Vermont
803	US:SC	South Carolina
812	US:IN	Indiana (southern)
814	US:PA	Pennsylvania (northwestern and central)
815	US:IL	Illinois (northern, except Chicago and Quad Cities)
816	US:MO	Missouri (northwestern, including Kansas City)
901	US:TN	Tennessee
902	CA:NS,PE,NB	Nova Scotia, Prince Edward Island, New Brunswick
913	US:KS	Kansas (northern half, including Kansas City, Kansas)
914	US:NY	New York (southern, including Long Island but excluding New York City)
915	US:TX	Texas (western, including El Paso)
916	US:CA	California (northern, but not including Sacramento)

Assignment plan

States with more than two NPAs
New York	212, 315, 518, 716, 914
Illinois	217, 312, 618, 815
Ohio	216, 419, 513, 614
Pennsylvania	215, 412, 717, 814
Texas	214, 512, 713, 915
California	213, 415, 916
Iowa	319, 515, 712
Michigan	313, 517, 616
States and provinces with two NPAs
Indiana, Kansas, Massachusetts, Minnesota, Missouri, Wisconsin, Ontario, Quebec

The number of central offices that could be effectively installed with a numbering plan with two letters and one digit for the central office code was expected to be approximately five hundred, because acceptable names for central offices had to be selected carefully to avoid miscommunication.^[9] States or provinces that required this many offices had to be divided into multiple smaller areas. Next to size, another important aspect was the existing infrastructure for call routing, which had developed during preceding decades independently of state or municipal boundaries. Since traffic between numbering plan areas would require special Class-4 toll switching systems, planning the divisions avoided cutting busy toll traffic routes, so that most toll traffic remained within an area, and outgoing traffic from one area would not be tributary to toll offices in an adjacent area.^[11][9] However, it was recognized already in 1930 that too little granularity in the allocation pattern introduced expensive traffic back-haul requirements, conceivably resulting in congestion of the routes to the centers.^[2]

Consideration of several assignment patterns resulted in the configuration that was publicized as a map in October 1947.^[7] The three-digit codes were assigned to numbering plan areas in seemingly random manner, avoiding consecutive, nearly-consecutive, or just very similar codes in neighboring numbering plan areas to avoid customer confusion,^[12] even when located in the same jurisdiction. This criterion was not always achieved, however. A more geography-based enumeration method had been examined earlier, but was discarded. Thus, it would not have been possible to locate the approximate geographic location of a numbering plan area by its code alone. The plan divided New York into five areas, the most of any state. Illinois, Ohio, Pennsylvania, and Texas were assigned four area codes each, and California, Iowa, and Michigan received three. Eight states and provinces were divided into two NPAs.

Rotary dialing telephones enabled customers to dial stations on their own exchange. The Bell Company allowed customers to dial direct to foreign exchanges beginning in 1951 in the US. However, switchboard operators were used for most long-distance calls until this became the norm in the 1960s and 1970s.

The pattern of this assignment of area codes is shown in the following tabulation. This method of arrangement, which is known to have been in use at Bell Laboratories,^[13] shows clearly that area codes were assigned not entirely in random order, but by filling the table in diagonal manner from the top left corner, containing the low-numbered area codes, toward the center and lower right corner. Such a pattern suggests that the designers intended to maintain the same degree of randomness in digits for the remaining, yet unassigned codes. The first area code (201) was given to the entire state of New Jersey, the state with the greatest population density in the nation. Despite its density, the state was not subdivided until about a decade later. In fact, in the group of single-NPA states, having the middle digit 0, all of the low-number codes were assigned to the mid-Atlantic states around New Jersey, i.e. the District of Columbia, Connecticut, Maryland, Delaware, and Rhode Island, all states in the top of the list of states with the greatest population densities.^[14]

Because telephones of this era, e.g. the Western Electric 302 desk rotary phone or the M3 354 wall telephone, were designed to send pulses or clicks to the central office's switching station, smaller digits were quicker to dial. This makes the fastest-dialing area code 212 (5 total clicks), followed by 312 and 213 (6 clicks). The area code 605 is much slower than these, at 21 clicks. Many large US cities generally had fast-dialing area codes with the middle digit 1. Touch-tone dialing, introduced later, eliminated the wait.

Single-NPA States and Provinces N N00 N01 N02 N03 N04 N05 N06 N07 N08 N09 2 † NJ DC CT MB AL WA ME ID   3 † MD DE CO WV FL SK WY 4 † RI NE AB GA OK MT 5 † AR KY OR LA NM 6 † MS AZ NH BC SD 7 † ND NV VA NC 8 † UT VT SC 9 † TN NB NS PE	Multi-NPA States and Provinces N10 N11 N12 N13 N14 N15 N16 N17 N18 N19 † † NY CA TX PA OH IL MN † † IL MI MO NY KS IN IA † † PA MA WI CA ON QC OH † † TX OH QC IA MI NY † † MN ON OH MI MA IL † † IA TX WI NY PA † † IN PA IL MO † † KS NY TX CA
Notes:     † Special dialing codes (not permitted as area codes)–Area codes could not begin with N=0 or 1, or end in 0

In this table, the assignments of the nine area codes to the Canadian provinces are highlighted by a blue background. The red fields are the NPAs that hosted the Regional Centers for toll-switching established in the General Toll Switching Plan of 1929:^[2] New York City (212), Los Angeles (213), Dallas (214), Chicago (312), St. Louis (314), and San Francisco (415) in the multi-NPA states, and in Denver (303) and Atlanta (404) in states with just a single area code each. These NPAs are clustered in the upper left corner, especially in the multiple-NPA table, but area codes 303 and 404 also fit neatly into the corresponding white positions of the N1X table (not Regional Centers), so that the Regional Center formed an almost closed block when ignoring the middle digit.

These assignments of these Regional Centers was complemented by assignments to the toll offices (in orange fields) in Detroit (313), serving the busy toll route to Toronto,^[15] and in Philadelphia (215), which had been chosen for the first cut-over into commercial service of a No. 4 Crossbar toll switch in 1943 for regional toll service.^[16]

The codes of the forms N00, N10, and N11, where N is 2 through 9, were not available for assignment as area codes at the time, but were reserved as special codes,^[9] leaving a total of 136 possible combinations. The series N00 was used later for non-geographic numbers, starting with intrastate toll-free 800 numbers for Inward Wide Area Telephone Service (WATS) in 1965.^[17] N10 numbers became teletypewriter exchanges,^[18] and N11 were used for special services, such as information and emergency services.

Implementation and expansion

For several years, area codes could be used only for Operator Toll Dialing by long-distance operators on routes between toll offices equipped with trunk code translation equipment. This absent, operators still had to rely on route operators and office-specific trunk codes, or employ the previous method of ringdown forwarding between intermediate operators.^[7] For entering the destination codes and telephone numbers into newly designed machine-switching equipment, long-distance operators did not use the slow rotary dials, but a ten-button key set, operating at least twice as fast, which transmitted multi-frequency (MF) tone pulses over regular voice channels to the remote switching stations.^[19][20] Such channels were incapable of transmitting the direct-current pulses of a rotary dial beyond a single link. All existing toll switching offices, many still using direct-control (step-by-step) methods, had to be supplemented with components to permit MF signaling and automatic route selection.

In December 1948, AT&T advanced the new long-distance technology with the cutover of new crossbar switching systems for toll-dialing in New York and Chicago,^[21] which added new technology to the No. 4 Crossbar Switching System, first installed in Philadelphia in 1943. This enabled operators to make calls directly to distant telephones without additional operators en route to some three hundred cities, and resulted in the handling of about ten percent of all Bell System long-distance calling by Operator Toll Dialing.^[19] On average, it took about two minutes for any long-distance call to be completed to its destination. As foreseen and stated in 1949, the target goal for call completion, after full implementation of the system across the nation, was one minute.

As new switching systems came online and design and manufacture of other routing infrastructure proceeded, the implementation of the new numbering plan advanced and several numbering plan areas were redrawn or added during subsequent years. In 1948, northern Indiana received an extra area code (219) for its Chicago suburbs by dividing area code 317.^[22] In 1950, southwest Missouri, with a new toll-center in Joplin, received area code 417, a change that provided more central offices in Kansas City. In 1951, the number of area codes grew to ninety: the State of New York gained area code 516 on Long Island, and Southern California received area code 714, to reduce the numbering plan area of Los Angeles.^[23]

Area code handbook issued by many telephone companies in 1962 to promote the newly introduced direct distance dialing.

By 1951, a new Class-5 switching system, the No.5 Crossbar, had proven successful in Media, Pennsylvania, and automatic message accounting (AMA) became available for billing telephone calls. The switching system was capable of handling the dialing of as many as eleven digits by subscribers.^[24] In Englewood, New Jersey, such a system was linked to a toll-class switch with the first commercial transistor circuitry that enabled the system to automatically translate area codes into toll trunk codes.^[25] On November 10, 1951, the system commenced a customer trial of direct distance dialing (DDD) from this single location in the country.

From the customer dialing experience in the Englewood DDD trials, Bell Laboratories engineers gained the confidence to predict that customers would use the new numbering plan with a reasonable degree of convenience and accuracy.^[26] After the success of these trials,^[23][27] expansion of the numbering plan accelerated with new crossbar systems and four new area codes in 1953, and seven in 1954. By the end of the decade thirty-one new area codes had been created in addition to the initial allotment of 1947.^[28] to satisfy the post-war surge in demand for telephone service.

While the first customer-dialed call using an area code was made in 1951, it took nearly fifteen years after the 1947 announcement of the new numbering plan that direct distance dialing was common in most cities. By then, some of the initial criteria for assignment, such as the 0/1 rule for single/multiple NPA assignments in a given state had to be abandoned by new requirements from population shifts and growth of communication services. In 1960, AT&T engineers, estimating that the capacity of the numbering plan would be exhausted by 1975,^[28] prepared for the next major advance in the evolution of the network by eliminating central office names, and introducing all-number calling (ANC). ANC, once supplemented by interchangeable central office codes during the 1970s, increased the number of central office prefixes possible in each numbering plan area from 540 to an eventual maximum of 792.

See also

• List of future North American area codes
• List of North American Numbering Plan area codes

References

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2. BSTJ 9: 3. July 1930: A General Switching Plan for Telephone Toll Service. (Osbourne, H.S.). July 1930.
3. Hosford, Howard L. (Winter 1943). "A Dial Switching System for Toll Calls". Bell Telephone Magazine. 22 (4): 230. Retrieved 2022-04-23.
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5. Annual Report for the Year 1945. American Telephone and Telegraph Company. 1946-02-15. p. 15.
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9. 'Notes on Nationwide Dialing (PDF) (Report). Department of Operation and Engineering, AT&T. 1955. Archived (PDF) from the original on 2014-10-14. Retrieved 2022-04-28 – via Exploding the Phone.
10. Shipley, F. F. (October 1945). "Nation-Wide Dialing" (PDF). Bell Laboratories Record (10): 368–372. Retrieved 2022-10-13.
11. W.H. Nunn, Nationwide Numbering Plan, Bell System Technical Journal 31(5), 851 (1952)
12. Erickson, Andrew (March 2021). "New In Town". American University Magazine. Retrieved January 7, 2022. (NANPA] relies on a tried-and-true methodology that considers factors like customer confusion.
13. Keevers, R. J. (1975-12-12). "North American Numbering Plan–The First Thirty Years". Bell Laboratories Memorandum No. 40978.
14. "Historical Population Density Data (1910–2020)". United States Census Bureau. 2021-04-26. Retrieved 2022-01-12.
15. H. S. Osborne, A General Switching Plan for Telephone Toll Service, Bell System Technical Journal Vol 9 (3), July 1930, p.431
16. Clark, A. B.; Osborne, H. S. (1952). "Automatic Switching for Nationwide Telephone Service". Bell System Technical Journal. 3 (5): 823. doi:10.1002/j.1538-7305.1952.tb01410.x. S2CID 51639224.
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18. AT&T, Notes on the Network (1968), Section 9
19. "Nation-Wide Toll Dialing Makes Its Bow" (PDF). Bell Laboratories Record. 27 (2): 71–72. February 1949. Retrieved 2022-01-20.
20. Moody, D. L. (December 1945). "Multi-Frequency Pulsing" (PDF). Bell Laboratories Record. 23 (12): 466–470. Retrieved 2022-01-20.
21. Guengerich, E. J. (Winter 1948). "Toll Dialing by Operators Reaches Some 300 Places". Bell Telephone Magazine. 27 (4): 228–237. Retrieved 2022-01-20.
22. "Nation-Wide System for Toll-Line Dialing" (PDF). Bell Laboratories Record. 27 (1): 29. January 1949. Retrieved 2022-01-20.
23. Baker, Ernest W. (Winter 1951). "Toll Dialing Is Expanding Throughout the Nation". Bell Telephone Magazine. 30 (4): 253. Retrieved 2022-01-20.
24. Korn, F.A.; Ferguson, James G. (1950). "Number 5 Crossbar Dial Telephone Switching System". AIEE Transactions. 69 (1): 244–254. doi:10.1109/T-AIEE.1950.5060147. S2CID 51637587. The Number 5 Crossbar System ... can be used for as few as four digits in a subscriber number, or as many as 11 digits may be used to code the called subscriber completely. (abstract)
25. P. Mallery, Transistors and Their Circuits in the 4A Toll Crossbar Switching System, AIEE Transactions, September 1953, p.388
26. Nunn, W.H. (1952). "Nationwide Numbering Plan". Bell System Technical Journal. 31 (5): 959. doi:10.1002/j.1538-7305.1952.tb01412.x. Retrieved 2022-01-20.
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