To help eliminate the confusion, the Bureau of Navigation, part of the U.S. Department of Commerce, began assigning three-letter call signs to American ships in early 1912, using the K prefix for ships on the Atlantic and Gulf coasts and W for ships on the Pacific coast and the Great Lakes; the reasons for choosing K and W, if any, are unknown. Shortly thereafter, at the London International Radiotelegraphic Convention, ranges of letters were assigned to each of the participating nations; in addition to W and most of the K range, the U.S. got the N prefix (to be used only by the navy).

On May 9, 1913, the U.S. Department of Commerce issued a 4-page document titled Radio Call Letters, which laid out the official policy in some detail. A couple of paragraphs bear particular mention:

3. (b) The combinations KDA to KZZ, with a few exceptions, are reserved for ship and coast stations on the Atlantic coast and Gulf of Mexico.

3. (c) The combinations beginning with W…are reserved, with a few exceptions, for ship and coast stations on the Pacific coast and the Great Lakes.

There you have it, clear as day: like the ships off the Atlantic coast, the land-based stations in the east were to have K designations, while stations in the west, like ships off the Pacific coast, were to have W designations. For entirely unknown reasons, though, these labels got swapped before they were implemented. Later that year, stations in the west began getting K call letters, while stations in the east got W call letters. A 1914 booklet of regulations titled Radio Stations of the United States codified the K-in-the-west, W-in-the-east practice, which was followed strictly thereafter, except when it wasn’t.