United States Callsign Policies

Thomas H. White -- January 1, 2023

Following is a summary of information I've uncovered about early United States radio callsign practices. The main text consists of selected call letter policy sections from Mystique of the Three-Letter Callsigns and K/W Call Letters In The United States. To this I've added background information and explanatory notes. Sections copied from the original articles are displayed in italics, while the additional information is in regular text.

Ship Visual Signal Letters (pre-radio)

Beginning in 1869, the U.S. government annually published an official List of Merchant Vessels of the United States, in which basic ship information was reported. The key identifier for each listed vessel was its "official number", however, in the mid-1880's the government also began assigning to ships optional visual identifiers, known as "signal letters". In the period through 1933 about 25% of U.S. ships were assigned signal letters. Moreover, the same 1884 act which authorized the issuing of visual identifiers would also be interpreted, in 1911, as mandating government oversight in assigning radio call letters to U.S. merchant vessels.

According to the 1893 edition of the annual vessel list, in 1856 the visual signaling system had been developed by a committee of the British Board of Trade, and
...what is now known as the "International Code of Signals" has since been adopted by the following nations for use of national as well as merchant ships: England, France, Germany, Russia, Brazil, Italy, Austria, Holland, Spain, Sweden and Norway, Denmark, Belgium, Portugal. Although in general use by vessels of the United States as well as other countries, our Government has not yet formally recognized the "International Code".
Under the provisions of the International Code of Signals, 1,440 four-letter combinations from GQBC through GWVT were reserved as ship identifiers for men-of-war and government vessels, while merchant ships drew upon 53,040 combinations from HBCD though WVTS. (Ship identifiers were always four different letters, i.e. no letter appeared more than once. Also, until 1901, there were only 18 letters, all consonants, in the International Code.) Displaying a ship's visual identifier required six flags -- the National Ensign, the Code Pennant, plus a letter flag for each character in the ship's four-letter identification. Because more than one country could assign the same four-letter identifier to a ship, the National Ensign was used to differentiate between vessels from different countries.

With the introduction of radio, ships began to use separate radio callsigns as identifiers for their transmitters. Until 1933 there was no relation between the visual signal letters and the radio callsigns assigned to U.S. ships. (By 1932 government ships were still being assigned 4-letter G--- visual signals, and in the U.S. non-government vessels had been issued sequentially-assigned visual signals from H--- through M---. In contrast, radio callsigns for U.S. non-government ships consisted exclusively of four-letter K--- and W--- calls). Finally, as of the 1933 list, with the widespread adoption of radio, the visual signal assignments were eliminated as no longer needed, although ships that had radio transmitters were now instructed they could employ their radio callsigns for signal letter purposes.

Early Radio Callsign Practices (pre-1912)

The use of identifying radio callsigns actually had its origins in wire telegraphy, where each station and operator along a telegraph line was assigned a short "call" or "signal". Because all early radio work was also done in telegraphic code, spelling out an operator's full name or location would have been cumbersome, so, following the landline telegraph practice, radio transmitter calls were assigned, usually from one to three characters and often based on geographic location or personal or ship names. Thus, stations "calling" each other were able to link up with a minimum of sorting out identities. Unfortunately, during this self-assigned era there were few standards, which resulted in problems when, say, two or more ships chose the same call. Unique identifiers, organized by national origin, were needed in order to keep track of exactly which vessel was in danger of visiting Davy Jones' locker.

According to Regulation IV in the Service Regulations of the 1906 Berlin International Wireless Telegraph Convention, radio transmitters for Coastal and Ship stations were to have call letters "distinguishable from one another and each must be formed of a group of three letters". However, reviewing the station lists, many stations were slow to make the switch to three-letter calls, and initially there seems to have been limited procedures for insuring unique calls. As early as 1905 most German stations were using unique three-letter calls, by 1908 most Marconi-operated stations worldwide began using three-letter calls starting with M, while U.S. Revenue Cutters began using three-letter RC- calls. Also, effective November 20, 1909 the U.S. Navy switched from a wide range of two-letter calls to three-letters starting with N. But through 1912 there were still duplicates and many stations using calls of just one or two letters, with little evidence of international coordination. (See, for example, the callsign list appearing in the U.S. Navy's January 1, 1912 edition of Wireless Telegraph Stations of the World).

Adoption of Nationally Assigned Prefixes

Each edition of the Commerce Department's Annual List of the Merchant Vessels of the United States included a section, "Part VI: Seagoing Vessels of the United States with Official Numbers and Signal Letters", which contained the list of unique visual signal identifiers that had been assigned to U.S. ships under the "International Code of Signals" system reviewed above. And, with the June 30, 1911 edition, this section for the first time also included a list of radio callsigns in use by U.S. ships. The radio calls in this initial list had generally been self-assigned by the radio companies operating the on-board transmitters.

The head of the Bureau of Navigation, Eugene Tyler Chamberlain, was disturbed by the number of duplicate and non-standard calls that appeared in the 1911 radio call list. Moreover, in the Call Letters section of the Annual Report of the Commissioner of Navigation to the Secretary of Commerce and Labor for the Fiscal Year Ended June 30, 1911, Chamberlain declared his intention to correct this problem:
The service regulations of the Berlin Radiotelegraphic Convention provide that every wireless ship or shore station shall have its own call letters (signifying its name), which must consist of three letters of the alphabet. [NOTE: Although U.S. representatives had signed this convention in 1906, the U.S. would not formally ratify it until April 3, 1912.] To prevent duplication of call letters is one of the duties of the International Bureau of Telegraphs at Berne, Switzerland, which has charge of general administrative work in the interest of wireless telegraphy. Call letters for American ships equipped with wireless have been assigned almost haphazard by the wireless companies. The Code List for 1911 (pp. 102-103) contains a list of 361 American merchant vessels and yachts, arranged according to their wireless call letters. This publication was for the convenience of shipmasters in signaling, but the list when assembled shows defects. In some instances three letters of the alphabet are used, in others a letter and a number, and in others a number alone. Where two letters are used, in 6 cases the same two letters have been assigned to three vessels, and in 25 cases the same letters have been assigned to two vessels. It is time that the United States brought its wireless code into conformity with that of the rest of the world, just as our code of signals by flags and pennants is the international code. There must be a central bureau to assign wireless call letters to merchant vessels and yachts to prevent duplications and to bring these call letters into accord with the system adopted by the rest of the maritime world. Under the three-letter call adopted by the Berlin convention, over 18,000 distinctive calls can be assigned without duplication. This number will suffice for the wireless ship and shore stations of the world for years to come. The two-letter system now employed by the wireless companies in the United States will soon exhaust itself. The Bureau of Navigation was charged by the act of July 5, 1884, with the assigning of signal letters to American merchant vessels, and is making arrangements to assign wireless call letters at the same time, and do for the wireless code the work it now performs for the international code.
The authority to assign radio call letters to ships, which Chamberlain claimed under the July 5, 1884 act, did not include land stations. (At this time the U.S. government had not yet started licencing radio transmitters). The date of the changeover to the new three-letter radio calls wasn't stated in the 1911 annual report, however, an October 25, 1912 General Letter from the Department of Commerce and Labor later referred to "the call letters assigned to American ship stations on July 1, 1912, by the Bureau of Navigation". (U.S. merchant ships were still using their old two-letter calls according to the January 1, 1912 edition of the Navy's Wireless Telegraph Stations of the World, but the new three-letter calls are listed in the radio call letter list in the June 30, 1912 edition of the Annual List of the Merchant Vessels of the United States, with K calls assigned to ships on the "Atlantic and Gulf Coasts", and W calls going to the "Pacific Coast" vessels.) In many cases, all that was done was to add a K or W in front of the ship's original two-letter call -- see 1911-1913 Ship Callsign Comparison Chart for more details. I don't know why K and W were chosen for the initial letters, or why the Bureau thought it necessary to split the assignments into two geographic groups -- they had not done anything similar with the visual signal letters. It is possible that W stood for "west", but that is pure speculation on my part.

These new three-letter K and W ship radio calls were assigned just prior to the signing of the 1912 London International Radiotelegraphic Convention. A note appearing with the June 30, 1912 lists stated that "Radio call letters of American vessels on the Atlantic and Gulf in the K series are subject to revision under international agreement". The modification that was reported shortly after the end of the London Convention was that, although all initial N and W callsigns were assigned to the U.S., only calls from KD through KZ were included in the U.S. allocation, so the U.S. ships that had been assigned KA- and KC- calls in the June 30, 1912 list had to be issued new ones -- see 1911-1913 Ship Callsign Comparison Chart for more details.

Then, with the passage of the Radio Act of 1912, the Bureau of Navigation was assigned the additional responsibility of licencing both ship and land radio stations, so it now also controlled call letter assignments for land stations, which had also traditionally used two-letter calls. And for commercial land stations the Bureau adopted the opposite policy from what it used for ship stations, i.e. land stations were issued three-letter K calls in the west and W in the east. In many cases, the process again consisted of adding a K or W in front of the station's original two-letter call, e.g. HA at Cape Hatteras, North Carolina became WHA, GO in Chicago, Illinois became WGO, and PH in San Francisco, California became KPH. And some land stations which already had three-letter calls just had their initial letter changed to meet the new international standard, e.g. MCC in South Wellfleet, Massachusetts became WCC. (For more information on the initial land station call letter assignments, see the July 1, 1913 issue of Radio Stations of the United States.)

(NOTE: A May 9, 1913 Commerce Department publication, Radio Call Letters, stated that, like ship stations, land stations would get W calls in the west and K in the east. The identical wording appears in the July 1, 1913 issue of Radio Stations of the United States, which was the first official station list issued after the U.S. began licencing stations. However, according to the station lists appearing in the latter publication, it is clear that the commercial land stations were actually being issued calls in the opposite pattern, i.e. K in the west and W in the east. And, beginning with the July 1, 1914 edition of Radio Stations of the United States, the wording was changed to reflect the correct pattern for land stations.)

The earliest reference to international call letter assignments that I know of appeared in the May 9, 1913 Radio Call Letters pamphlet. According to this:

The London International Radiotelegraphic Conference made a partial allotment of call letters among nations which signed the convention and the International Bureau at Berne, with the consent of such nations, has modified and added to this assignment of call letters by circular of April 23, 1913. The distribution of call letters among nations thus authorized is printed below for the guidance of operators of all stations, ship and shore, of the United States.
A.............All to Germany and protectorates.
B.............All to Great Britain.
CAA to CMZ....Not yet assigned.
CNA to CNZ....Morocco.
COA to CPZ....Chile.
CQA to CQZ....Monaco.
CRA to CTZ....Portugal and colonies.
CUA to CUZ....Not yet assigned.
CVA to CVZ....Roumania.
CWA to CWZ....Uruguay.
CXA to CZZ....Not yet assigned.
D.............All to Germany and protectorates.
EAA to EGZ....Spain and colonies.
EHA to EZZ....Not yet assigned.
F.............All to France and colonies.
G.............All to Great Britain.
HAA to HFZ....Austria-Hungary and Bosnia-Herzegovina.
HGA to HHZ....Siam.
HIA to HZZ....Not yet assigned.
I.............All to Italy and colonies.
J.............All to Japan and possessions.
KAA to KCZ....Germany and protectorates.
KDA to KZZ....United States.
LAA to LHZ....Norway.
LIA to LRZ....Argentine Republic.
LSA to LWZ....Not yet assigned.
LXA to LZZ....Bulgaria.
M.............All to Great Britain.
N.............All to the United States.
OAA to OFZ....Not yet assigned.
OGA to OMZ....Austria-Hungary and Bosnia-Herzegovina.
ONA to OTZ....Belgium and colonies.
OUA to OZZ....Denmark.
PAA to PIZ....Netherlands.
PJA to PJM....Curacao (Dutch).
PJN to PJZ....Surinam (Dutch).
PKA to PMZ....Dutch East Indies.
PNA to PZZ....Not yet assigned.
Q.............Reserved for code abbreviations.
R.............All to Russia.
SAA to SMZ....Sweden.
SNA to STZ....Brazil.
SUA to SUZ....Egypt.
SVA to SZZ....Greece.
TAA to TMZ....Turkey.
TNA to TZZ....Not yet assigned.
UAA to UMZ....France and colonies.
UNA to UZZ....Austria-Hungary and Bosnia-Herzegovina.
VAA to VGZ....Canada (British).
VHA to VKZ....Australian Federation (British).
VLA to VMZ....New Zealand (British).
VNA to VNZ....South African Union (British).
VOA to VOZ....Newfoundland (British).
VPA to VSZ....British colonies not autonomous.
VTA to VWZ....British India.
VXA to VZZ....Not yet assigned.
W.............All to the United States.
XAA to XCZ....Mexico.
XDA to XZZ....Not yet assigned.
YAA to YZZ....Not yet assigned.
ZAA to ZZZ....Not yet assigned.
Notes: The full name of the "International Bureau" referred to above was "The International Bureau of the Telegraph Union". A current list of international callsign allocations is available at Call Sign Prefixes.

Under international agreement unique initial letters were allotted among the various nations. The July 1, 1914 edition of Radio Stations of the United States records the contemporary practices for allocating calls for sea and commercial land stations, which at this time were few enough so that all could be given three-letter calls:
The call letters assigned to the United States are all combinations (676) beginning with the letter N and all (676) beginning with the letter W, and all combinations (598) from KDA to KZZ, inclusive. [NOTE: KAA-KCZ was allocated to Germany at this time, and was not assigned to the United States until 1929.] The total number of international calls is thus 1,950, and these are reserved for Government stations and stations open to public and limited commercial service. All combinations beginning with the letter N are reserved for Government stations and in addition the combinations from WUA to WVZ and WXA to WZZ are reserved for the stations of the Army of the United States.

The combinations KDA to KZZ, with a few exceptions, are reserved for ship stations on the Atlantic and the Gulf of Mexico and for land stations on the Pacific coast. The combinations beginning with W (except WUA to WVZ and WXA to WZZ as already indicated) are reserved, with a few exceptions, for ship stations on the Pacific and Great Lakes and for land stations on the Atlantic and Gulf coasts and in the Great Lakes region.

Amateur and Special Land stations fell into a separate callsign scheme. In fact, they did not qualify for "international" calls, and the International Bureau at Berne was not notified of their existence. The United States was divided into nine Radio Inspection Districts and these stations received calls consisting of their district number followed by a pair of letters. Regular amateurs received calls whose first letter was from A through W, for example, 8MK. (The 1914 Radio Stations of the United States noted that "The three items-a given figure first, followed by two letters of the alphabet-thus may be combined in 598 different calls, which will probably suffice for the amateur sending stations in most districts for some time to come".) Among the three licence classes known collectively as Special Land Stations, X was reserved as the first letter for stations holding Experimental licences (e.g. 1XE), Y designated stations holding a Technical or Training School licence (e.g. 9YY), and Z went to stations operating with Special Amateur licences (e.g. 8ZZ).

Post-1912 Refinements

The Bureau of Navigation, a division of the Department of Commerce, regulated United States radio until the 1927 formation of the Federal Radio Commission. In 1934 the Federal Communications Commission succeeded the FRC. Understandably, the various agencies occasionally found it necessary to refine callsign practices.

World War I also had a disruptive effect. German submarines did much to popularize radio among American ships as wireless, formerly an expensive option, became a life-or-death necessity for making the Atlantic run. Unfortunately, there weren't enough three-letter calls to go around. The obvious solution was more letters, and four letter KE-- signs became the predominate issue for the rapidly expanding ship service, generally issued on a first come, first served basis in alphabetical order. The department, apparently noting the existence of the Panama Canal meant ships might show up on either coast, no longer tried to give ship calls that differentiated between the east and west coasts.

The less numerous land stations continued to receive three-letter calls, as turnover provided a reserve pool. Actually "turnover" is in some cases a euphemism. In the July, 1928 Radio Broadcast magazine
, Broadcast Station Calls With a Past by William Fenwick reviewed a few land stations, including broadcasters WSB Atlanta and KLZ Denver, which received calls that became available with the demise of the ships that had used them. Because superstitious seafarers objected to being issued the calls "used by that ship which went down with all hands last month", these "tainted" calls were quietly issued to unsinkable land stations.

The KDKA Anomaly

Showing partiality to vowels, the next major blocks drawn upon for ship stations were four letter KI--, KO--, and KU-- calls. After exhausting the vowels, and with KA-- to KC-- not yet assigned to the United States, the first available consonant, KD--, was drafted for ships beginning June 1920. At this point an anomaly occurred. The Bureau, perhaps caught up in a burst of egalitarianism, began assigning the last of the KU--, and the new KD-- calls to most stations, whether land or sea. The result, on October 27, 1920, was that a new Westinghouse station in East Pittsburgh, Pennsylvania, KDKA, was sandwiched between the ships Montgomery City (KDJZ) and Eastern Sword (KDKB). The "KD-- for everyone" policy continued until April, 1921, when the original three-letter land station policy was reinstated. This meant that, in May, 1921, when the second Westinghouse broadcast station, WJZ in Newark, New Jersey (now WABC, New York City) was authorized, the original call policy had been restored. Much speculation has been made about the unique status of KDKA's call, but this uniqueness actually is just a fluke, due to the fact that no other surviving broadcaster was licenced during this short anomaly. Had KDKA been licenced a few months earlier or later it most likely would have gotten a three-letter W call like everyone else.

[NOTE: two other land stations licenced during this anomaly, KDPM Cleveland, Ohio, and KDPT San Diego, California, both originally non-broadcasting service stations, later transferred to the broadcast service but were eventually deleted.]

I haven't found any explicit reference to the temporary switch to predominantly four-letter K assignments for commercial land stations, but the pattern is clear when reviewing the new station grants listed in the Commerce Department's monthly Radio Service Bulletin. By my count, in the 8/1/1919 through 6/1/1920 RSBs there were 39 non-government land station grants, with 33 (85%) of these getting three-letter K or W calls. But during the next month fully 15 of the 21 grants received four-letter K calls, and, for the entire 7/1/1920 through 5/2/1921 period, 49 (73%) of the 67 grants received four-letter K calls, including KDKA. Then there was an abrupt switch back to three letters, with all 18 of the land station grants in the 6/1/1921 to 9/1/1921 period getting three-letter assignments.

Dawn of the Four Letter Calls

The flood of broadcasting service authorizations that began in earnest in December of 1921 served to overload the recycling three-letter land station calls. Before the crunch the Bureau was able to assign three-letter callsigns to about 200 broadcasters.

It was the more saturated East that was the first to feel the pinch. On April 4, 1922 an application from the Times-Picayune of New Orleans broke new ground with the assignment of WAAB (now WJBO, Baton Rouge) as its call. [NOTE: WAAA was skipped as no sign was permitted with the same letter three times in a row.] The progression continued in alphabetical order, with "A" fixed as the third letter, i.e. WAAB, WAAC, WAAD... WBAB, WBAC... etc. This explains why so many pioneers such as WBAP Fort Worth, Texas, WCAU Philadelphia (now WPHT), WEAF New York City (now WFAN), WHAS Louisville, Kentucky, WKAR, East Lansing, Michigan, WMAQ Chicago (now WSCR), WOAI San Antonio, and WTAM Cleveland share this same middle letter. In later years it became the norm for broadcasters to ask for distinctive calls. However, if they had no preference they were assigned calls from blocks used for a variety of radio services. Starting April of 1923 calls centering on "B" were issued, including WBBM Chicago, WCBM Baltimore, Maryland, WLBL Stevens Point (now Auburndale), Wisconsin and WMBD Peoria, Illinois. (WLBL had requested "WLOL", but was told it had to accept a W-B- call). In mid-1928 there was a jump to the middle of the W-D- block, which yielded WHDH Boston (now WEEI) and WRDW Augusta, Georgia. W-E- calls followed beginning in early 1931, including WDEV Waterbury, Vermont, WEEU Reading, Pennsylvania, and WFEA Manchester, New Hampshire. In 1934, W-F- calls started to be assigned, including WMFJ in Daytona Beach, Florida.

The West held out until May 8, 1922, when western broadcasters started sharing the four-letter ship blocks. KDYL in Salt Lake City was both the first authorization and last survivor of this group. When it became KCPX (now KNIT) December 21, 1959 all thirty-three KD-- authorizations from this switchover had either expired or changed calls. The KF-- block, begun June 1922, boasts a few more noteworthies, including KFBK Sacramento, California, Doc. Brinkley's infamous KFKB, KFNF Shenandoah, Iowa (now KYFR), KFQD Anchorage, Alaska, and KFYR Bismarck, North Dakota. The KG-- group was tapped July 1926: KGEZ Kalispell, Montana and KGFX Pierre, South Dakota are two that survive to this day. (A ship station was not as fortunate. KGOV was assigned to the Morro Castle, which went on to burn spectacularly off the New Jersey coast in 1934. However, surprisingly KGOV is currently unavailable for use by broadcasting stations, since it is technically still assigned to the ship, according to the FCC's online Call Sign Search page). KH-- calls were reserved, beginning in 1927, for a new service category: Commercial Aircraft Stations. Surprisingly this group included a short-lived broadcast authorization, KHAC, issued in late 1927 to Flying Broadcasters, Inc. in San Francisco, for "Airplane (unnamed)". The KI-- block was drafted in early 1932, which resulted in KIEV Glendale, California (now KRLA), followed over the next few years by such stations as KIUL Garden City, Kansas, KIUN Pecos, Texas, and KIUP Durango, Colorado.

Original K/W Dividing Line

In the early teens most non-amateur land stations engaged in ship-to-shore communication, and were found clustered along the coasts. As other services were developed stations crept inland, and a dividing line between the western K's and eastern W's was needed. As noted earlier, coastal land stations in states along the Atlantic and Gulf coasts, including Texas, received W calls. Thus, using the Texas-New Mexico border as a starting point and heading north, the original boundary ran along the eastern borders of New Mexico, Colorado, Wyoming, and Montana.

I've never found any official specification for the original land station dividing line between the K's in the west and the W's in the east. However, there were enough inland stations to require some sort of continental dividing line. My working theories were:
  1. A specific line of longitude.

  2. The division between the mountain and central time zones.

  3. Some sort of grouping of the nine Radio Inspection districts.

I took a graphical approach, i.e., I bought a big map of the United States, and plotted the land station assignments reported in the Radio Service Bulletin from 1915 through 1923. From this it became clear that none of the above three theories was correct -- two of the radio inspection districts were split between K and W calls, (compare Map of the K/W Division with Radio Inspection Districts Map) and the calls didn't break out by longitude or time zone.

West Texas proved to be particularly informative. Stations in El Paso got W calls, even though El Paso is west of Colorado Springs and Denver, Colorado, and Laramie, Wyoming, all of which got K calls. The overall pattern: during this period virtually all the inland stations in Texas, Oklahoma, Kansas, Nebraska, and the Dakotas got W calls, while all the land stations in Montana, Wyoming, Colorado and New Mexico got K calls. Therefore, I've concluded that the original dividing line was based on state boundaries. Because it was a Gulf coast state, Texas got W calls. Then the line went north to the Canadian border, running between the states listed above.

Switchover to the Mississippi River K/W Dividing Line

Reviewing the stations on the AM band, many people have noticed that some of them have the "wrong" first letter for the side of the Mississippi River on which they are located. During the century of call letter assignments for AM stations, I came up with six categories of non-conforming stations:
  1. Stations located east of the Mississippi which were assigned calls from the KD-- ship block, instead of W--, during a June 1920 to April 1921 anomaly. (For some reason, during this anomaly almost all new land stations, east and west, got KU-- or KD-- four-letter calls. This included two broadcasting stations that just happened to be first licenced during this time: KDKA and KDPM).

  2. Stations west of the Mississippi River that were licenced before the late January 1923 boundary shift, and were located in the slice of W territory that existed west of the Mississippi prior to the shift. (Originally about 170 stations, not including Minnesota and Louisiana. However, due to very high deletion rates plus later call changes, only eleven of these original calls survive: WEW, WHB, WKY, WOC, WOI, WBAP, WDAY, WJAG, WNAX, WOAI, and WTAW).

  3. Portable stations (prior to 1928), which got W call letters because their original owners were located east of the Mississippi, but settled in a permanent home west of the Mississippi. (Four stations: WBBZ, WIBW, WLBN, and WMBH. There are no examples of a portable crossing in the other direction, i.e. no K portables "anchoring" in W territory).

  4. Regular stations that changed their community of licence to the other side of the K/W divide. (Eight stations: KFKX, KSGM, KWEM, WKBB, WPLX, KOTC, KQQZ and WGNU. NOTE: This omits Louisiana and Minnesota.)

  5. Owner requests--examples: WACO in Waco, Texas; WDBQ in Dubuque, Iowa; WMT (Waterloo [Iowa] Morning Tribune).

  6. Assigned by the Government--three stations. KTGG in Spring Arbor (later Okemos), Michigan reportedly got a "K" callsign because someone at the FCC thought that the "MI" postal code stood for Missouri, a west-of-the-Mississippi state. Also, two additional call assignments appear to have been selected by government regulators: KYWA Chicago, a booster station for KYW, and KOP, licenced to the Detroit Police Department.
Finally, there are about a dozen stations for which I can not come up with any apparent reason--perhaps someone momentarily forgot about the policy, or where the boundary line was, or maybe I just need to do more research. The most prominent of these "undocumented" stations are KQV Pittsburgh, KSD Saint Louis (assigned before the boundary shift--now KTRS), and KYW Chicago (later Philadelphia and Cleveland).

Although KOP clearly was specially selected for the Detroit Police Department, to me it seems most likely that, instead of it being a requested call, someone within the Department of Commerce chose it, as a joke. I'm not sure that police in this era really liked being called "cops", or that they knew enough about radio to request call letters. But this is pure speculation -- eventually I want to get the original Form 761s for the KOP and KYWA applications and see they have any additional information. L. A. Corridon

Side note: according to a photograph appearing on page 734 of the October, 1925 Radio Broadcast, at that time L. A. Corridon was the person responsible for selecting callsigns.

Finally, there are about a dozen stations for which I can not come up with any apparent reason--perhaps someone momentarily forgot about the policy, or where the boundary line was, or maybe I just need to do more research. The most prominent of these "undocumented" stations are KQV Pittsburgh, KSD Saint Louis (assigned before the boundary shift--now KTRS), and KYW Chicago (later Philadelphia and Cleveland).

Although I haven't been able to find out the exact date that the K/W boundary for land stations switched to the Mississippi River -- the change was barely reported in the press -- I have been able to narrow it down to late January, 1923. In reviewing the call letter assignments for broadcasting stations in the original W-west-of-the-Mississippi territory, it appears that the change took place sometime between January 19th and January 29th. The first date is when the State Normal School in Mayville, ND was assigned WRAC and the Taylor Radio Shop in Marion, KS was assigned WRAD. These apparently were the last two stations to fall under the old scheme. The next call assignment to a broadcast station located in the original W-west-of-the Mississippi territory that I am aware of did not come until ten days later. This station, licenced to Fred Mahaffey, Jr., in Houston, TX, was assigned KFCV, and from this point on K calls were issued to virtually all stations west of the Mississippi. (KFCV was deleted January 5, 1925).

I've only found a few short contemporary references to the adoption of the Mississippi River dividing line, including one paragraph writeups in the April, 1923 issue of The Wireless Age and the June, 1923 Radio News.

Northern K/W Boundary Demarcation

The source of the Mississippi River is in upper Minnesota, so using it as the K/W boundary leaves a gap in the northern part of the state. In 1987 the Federal Communications Commission noted that the current staff practice was to define the remainder of the boundary as "a line from the headwaters of the [Mississippi] to a point [at the Canadian border] just east of International Falls".