The origins of the amateur radio call sign districts can be traced to 1912 when the Department of Commerce, Bureau of Navigation created nine radio inspection districts to enforce the 1912 Act to Regulate Radio Commerce. Primary foci of the act were to regulate ship to ship and ship to shore radios including those used on the Great Lakes. As a result, all nine offices where located at port cities where the inspections would be concentrated. They were:
- Boston, MA
- New York, NY
- Baltimore, MD
- Savannah, GA
- New Orleans, LA
- San Fransisco, CA
- Seattle, WA
- Cleveland, OH and then later Detroit, MI
- Chicago, IL
Take note that there were only nine districts defined at that time. It is also noteworthy that the first seven numbered offices were located at ocean port cities while Cleveland/Detroit and Chicago are Great Lake ports. If you look at the numbering, you can see the ocean port districts were numbered starting in the east with Boston as 1 and moving around the map in a clockwise fashion to the west ending with Seattle as 7. Then the two non-ocean, Great Lake port locations were assigned the remaining two numbers in an east to west fashion as well. This is not a documented methodology but it is my analysis as to the apparent logic of the numbering scheme.
On July 1, 1928 Alabama was transferred from the 5th district to the 4th district. Over time, other states were moved between districts and some that were originally split between districts were resolved to their present day districts.
The current form of the Amateur Radio call signs came to be in 1928 as a result of the International Radio Convention of 1927. While the provisions of the convention did not come into force until January 1, 1929, the Radio Division of the Department of Commerce announced that it wished to publish the "new" style of amateur radio call signs in June of 1928. The 1927 convention assigned the W and K prefixes to the US for use for amateur radio call signs (N was also available but the Navy used this at the time). The June 1927 edict was that all amateur radio stations in the continental United States add a W prefix to their call while those in Alaska, Hawaii, Porto Rico, and the US Virgin Islands should add a K prefix to their call. Thus a station with a call of 4ABC would immediately become W4ABC if it was on the continental United States.
In 1945-46, it was determined that the 9th District served too large of an area so Missouri, Kansas, Colorado, Iowa, Nebraska, South Dakota, and North Dakota would be grouped under a new tenth district. For purposes of call signs, this district was designated as 0.