Why W and K for US radio and TV stations?

One of those mundane questions I’ve long wondered about, but never bothered to look up, is why virtually all the radio and television stations in the US start with either the letter W or K

From https://www.primermagazine.com/2010/field-manual/know-it-all-w%E2%80%99s-and-k%E2%80%99s-the-history-of-radio-and-television-call-letters Primer Magazine: “In 1912, several countries attended a conference centered on the subject of ‘International Radiotelegraphs.’ One of the biggest things to come out of this gathering was the assignment of certain letters to certain countries, to identify their radio signals – America was given W, K, N, and A (fun fact: Canada got ‘C’ and Mexico got ‘X’).”

But why those particular letters has seemingly been lost. (A for America?)

“While N and A were chosen for American military radio stations, W and K were designated specifically for commercial use. Stations were allowed to choose the letters that followed the K or the W, and the combination was allowed to be three or four letters in length.”

Initially, the K stations were to the east and the W stations were to the west. Thus one can find early stations such as KDKA out of Pittsburgh, PA, established in 1920. By 1926, the Federal Communications Commission codified the idea of having four letters, but stations with three didn’t need to change.

From http://earlyradiohistory.us/kwtrivia.htm Warly Radio History:

“The original K/W boundary ran north from the Texas-New Mexico border, so at first stations along the Gulf of Mexico and northward were assigned W calls. It was only in late January, 1923 that the K/W boundary was shifted east to the current boundary of the Mississippi River. With this change, K’s were assigned to most new stations west of the Mississippi; however, existing W stations located west of the Mississippi were allowed to keep their now non-standard calls.”

This page has more information on the topic than most mortals would want to know, such as the K/W exceptions and other trivia. For instance, some break the rules by owner requests -examples: WACO in Waco, Texas; WMT (Waterloo [Iowa] Morning Tribune).

The page was compiled on 1 January 2017, so it’s quite recent.

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