Long a source of confusion for researchers, the plan for aircraft licenses and their display was actually grounded in logic, short-sighted though it may have been. A summary, digested in chronological form from William T Larkins' work in British Air Pictorial in 1954, is presented in hopes of clarifying the waters a bit:

1919 = The Convention for the Regulation of Air Navigation, as part of the October 1919 Peace Conference, created the system of international identification still in use that sets the first letter(s) as country of origin: N for United States, D for Germany, G for Great Britain, SE for Sweden, etc. This system was in use for seven years before it was formally ratified by our government.

1921 = In July the National Aircraft Underwriters Association, a service organization for the insurance industry, established a five-letter licensing code, but this system was voluntary with no governmental teeth in it. Because of indifference from manufacturers (only 33 planes were registered by the end of 1922, and it's doubtful if that number exceeded 50). It was history by 1925, but some aircraft of that period appeared as N-ABCA, N-ABCB, etc; see below.

1926 = In May the first real attempt at organization came with the federal Air Commerce Act that went into effect in January 1927. In this system a class letter C, S, or P was to be added, denoting Commercial, State, or Private. C specified approved (airworthy) airplanes used in commerce and the air mail, but this was amended in 1930 to include any aircraft meeting minimum government airworthiness requirements regardless of its use. S was for state- or federal-owned planes, with most all states requiring aircraft operated within their boundaries to bear an NC number (Oregon, where much flying activity took place, was a notable exception), but this was dropped in 1937. P only lasted until March 1927 to sort out private aircraft from C and S (no example of an NP designation was located). A limit of five numbers seemed adequate at the time for present and future aircraft, but these were all taken by 1929!

    "Identified Aircraft" was the term used to designate aircraft that did not meet minimum airworthiness requirements, and it was possible to register such an aircraft until March 1939. These would wear IMA (Identification Mark Assignment) numbers, usually without the N.

1929 = A new plan was to approve three numerals, and a suffix: E, H, K, M, N, V, W or Y. Not surprisingly, these new blocks were used up by the end of 1934.

    Class prefixes R and X for Restricted and Experimental aircraft were established. A class prefix of G identified Gliders until it was canceled in 1937, with sailplanes and gliders placed the same bag as powered aircraft. The use of the letter N was optional at this time for aircraft flown within the nation's boundaries.

1935 = Visionaries stepped in and claimed an increase to five numerals would surely do it. This opened up a block from 10000 to 99999, but these, too, showed signs of being gobbled up by the war years.

1938 = The Civil Aeronautics Authority (CAA) was established as an independent agency and, in 1940, was split into two parts—the Civil Aeronautics Board, to handle rule making, regulation, and accident investigation, and the Civil Aeronautics Administration, to take care of licensing and certification, airway development, and safety enforcement. By the outbreak of war, CAA had also assumed control of landings and takeoffs at airports. The Federal Aviation Agency (FAA) would not arrive until November 1, 1958.

1946 = Blocks of three and four numerals with the other letter suffixes (except number-lookalikes I and O) were added, and 46000 to 79999 were generally reserved for war-surplus aircraft. A class prefix L for Limited type certification went into effect, lasting only until 1948.

1948 = Class prefixes of C, R, and X were eliminated in December, and only the N was used.

1953 = Double suffixes with three numerals were authorized in March.
    None of these rules was carved in marble, and exceptions were manifold. A notable example is a de Havilland DH-4 that earned the very first license in its duties with the Department of Commerce. It proudly wore N1, even though to abide by its owner's rules it should have been NS1. Because of DoC's practice of reassigning numbers after the sale, destruction, or export of an airplane to another, N1 showed up later on a government Northrop Alpha 2, a Ford 5-AT, a Lockheed 12-A, and a DC-3!

    In other instances, a number might be borrowed temporarily by a manufacturer from an inactive company hack for use on a prototype until it had its own license, or a "blue-sky" number might be painted on a new model for photographic or publicity purposes.

    Special-request registrations became popular, accounting for the many low-number- plus-suffix registrations, especially after World War Two. If one had the $10.00 fee, one could have just about anything, as long as it was available.

    With batch allocations by CAA to regional offices for areal distribution, numbers became cloudy as a logical reference tool, indicating where airplanes were licensed, and not where or when they were built. Additionally, some batches were issued to large manufacturers, which explains how Douglas cornered the N30000 market.

Civil registrations using the Underwriters Laboratories all-letter system, 1921 to 1923:

N-AABA - Colonial Air Transport (Fokker Universal)*

N-AABB - Colonial Air Transport (Fokker Universal)*

N-AABC - Colonial Air Transport (Curtiss Lark)*

N-ABCA - John M Larsen, New York (Avro 504K)

N-ABCB - LMC Drilling Company, Wichita (Laird Swallow)

N-ABCC - Akers Airphoto Corporation, Chicago (Avro 504K)

N-ABCD - Ninimo Black Airport Corporation, Chicago (Laird Swallow)

N-ABCE - John A Hambleton, Baltimore (Avro 504K)

N-ABCF - Loening Aero Engineering Corporation, New York (Loening Air Yacht)

N-ABCG - Diggins Aviation Company, Chicago (Curtiss JN-4)

N-ABCH - Diggins Aviation Company, Chicago (Curtiss JN-4)

N-ABCI - Aero Club of Minneapolis (Curtiss JN-4C)

N-ABCJ - John C Metzger, Chicago (Curtiss JN-4)

N-ABCK - David L Behncke, Forest Park IL (Curtiss JN-4)

N-ABCL - David L Behncke, Forest Park IL (Curtiss JN-4)

N-ABCM - E Hamilton Lee, Chicago (Curtiss JN-4D)

N-ABCN - A W Stephenson, Miles City MT (Curtiss JN-4D)

N-ABCO - Chicago Tribune, Chicago (Curtiss C-6)

N-ABCP - Brooks, Banks & Smith Corp, Framingham MA (Avro 504K)

N-ABCQ - C E Lessong, Chicago (Standard J-1)

N-ABCR - R S Thompson, Oakland CA (Standard J-1)

N-ABCS - Northbird Aviation Co, Ketchikan AK (Curtiss MF)

N-ABCT - H P Ayres, Pittsburgh PA (Curtiss MF)

N-ABCU - Vincent Astor, New York (Loening Air Yacht)

N-ABCV - Harold S Vanderbilt, New York (Loening Air Yacht)

N-ABCX - Mrs K LaParle, Chicago (Curtiss MF)

N-ABCW - George E Weaver, Chicago (Waco 4)

N-ABCY - Triangle Airways (Curtiss MF)

N-ABCZ - Continental Motors (Judson-Kantner F Boat)**

N-ABDA - B D Burley, Chicago (Curtiss JN-4)

N-ABEA - L B Coombs, New York (LWF G-3)

N-ABFA - E P Hall (de Havilland DH-4)

N-ABGA - J Sorenson, Thomson IL (Curtiss JN-4D)

N-ABHA - T J Junker, El Dorado KS (Laird Swallow)

N-ABIA - G Mosny Jr, Indiana Harbor IN (Aeromarine 39-B)

N-ABJA - R R Ferguson, Chicago (Lincoln Standard)

N-ABKA - Anna M Parker, Hazelcrest IL (Curtiss JN-4D)

N-ABLA - American Airways, Cleveland (Curtiss H2-SL)

N-ABMA - Antone Brotz, Chicago (Curtiss JN-4D)

N-ABNA - Edward Hubbard, Seattle (Boeing B-1)

N-ABOA - Great Lakes Aviation Co, Cleveland (Lincoln Standard)

N-ABPA - Antone Brotz, Chicago (Lincoln Standard)

N-ABQA - W F Bridgeman, Ottumwa IA (Lincoln Standard)

N-ABSA - Victor Dallin (Laird Swallow)

N-AFOR - Stout Airplane Co, Detroit (Stout Air Pullman)

N-BMUL - R W Schroeder, Chicago (Standard J-1)

N-CAED - Spanish River Pulp & Paper Co, Ontario, Canada (Dayton-Wright FP-2)***

N-MAAB - William Eaton Jr, Boston (Travel Air C-6)

N-XAAA - Walter Becker, Newark NJ (de Havilland DH-6)

NOTE: Underlined letters on aircraft first specified those privately owned, but this idea was apparently soon dropped.

* William T Larkins explains that these three planes of the 1926 period were found, but it is unknown if the registrations were assigned by a renewed Underwriters Register, or if they were the private airline's own system.

** According to John W Underwood, Ross Judson ordered a 1922 Loening Air Yacht, which was issued UL registration N-ABCZ, which was later applied to the Judson-Kantner.

*** How a non-USA owner got in here is unexplained. Likely the "C" in the license was intended for Canadian use, as it did not show up again.

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Civil Registrations, 1927-1947