My search-fu is failing, possibly from an exhausting day at work so here goes:
Back before most of us were born radio stations here in the USA were finally designated with prefixes W for east of the Mississippi River and K for those west of it with a few notable exceptions. I realize the X prefix was used also.
Now for my question.
Why “W” and why “K”?
Was this a Morse Code kinda thing? .-- for W and -.- for K?
Idle minds want to know.
this was arrived at by international treaty.
the USA got to use call signs starting with AA to AL, K, N and W for radio. only W and K are used for broadcast radio.
The Master Speaks (prodded, if memory serves, by SD poster WhiteTho):
X was never a USA call sign letter.
in North America
XA–XI Mexico
XJ–XO Canada , their broadcast calls start with C.
Thanks.
Still unanswered is why W is east, and K is west.
Probably best commented by Cecil about civil servants and bureaucracy:
“What happened is not known, but one suspects a memo stating in essence, all right, you morons, enough is enough.”
Yes I knew Mexico had call prefixes with X. I thought that originally X meant experimental.
So call letters AA-AL are used for what here? And are they assigned specific frequencies? The FCC website is confusing and my brain is tired. 
PS You guys are great!
Well, one example I know of would be ham radio operator hobbyists. When I was a young child in the late 1950s-early-1960’s, my big brother was one. He was K6OAF. One could (and he did) even get automobile license plates with your call sign back then, long before “personalized license plates” were otherwise a thing.
ETA: Okay, that began with K, not A. Still, it’s an example of radio usage other than the usual broadcast station stuff.
radio services like ham radio, business radio and telemetry (remote data and control) all get call signs.
The AA and other A series are used by hams, possibly other services as well.
X was often in the call sign of early experimental stations but not the first letter.
most radio licenses are assigned a specific frequency(s).
in ham radio you are allowed to use any frequency in a range of frequencies (band) based on your level of license (higher qualification license allows more frequency range).
Although K in general is west of the Mississippi and W is east of the Mississippi, there are exceptions. KDKA is in Pittsburgh, and claims to be the first commercial radio station in the US. And WHO is in Des Moines.
Got that and understand.
Just wondering where the W and K came from. OK - International treaty.
Would have loved to see it:
“What do mean W? We want a K also!”
“We want a X! And a couple of XX’s while we are at it!”
“Yah. C belongs to us.”
“No. Marconi invented this. All alphabet belongs to us.”
Some radio stations in Newfoundland & Labrador still use the VO prefix Newfoundland was assigned back when it was an independent dominion.
I always speculated that W and K were particularly well-fitted to give to an English-speaking country, given the curiosity of those letters in Romance languages.
That is a copy from ten years ago, and I update the page on my site annually, so if you are interested in what has changed in the last 10 years (ans: not much) then here is a link to the January 1, 2015 edition: K/W Call Letters in the United States. And to recap what the others have said, the assignment of K and W seems to have been completely random, with no significance behind their selection.
Someone told me a long time ago is K and W are silent letters in English and that’s why they’re chosen. But I’ve never heard of a radio station that didn’t spell out its name and omit the W or K from the pronunciation, so I call “bullshit” on that theory.
My hometown radio station was KDIO (Ortonville MN) which I thought was an interesting rhyme for RADIO.
Besides KDKA Pittsburgh that Cecil mentioned there is also KYW Philadelphia which is currently shown as a CBS station, which it wasn’t when I was growing up. I have a dim memory, it was NBC and WCAU was the CBS affiliate (it still is; I just googled it). When KYW started a TV station, they wouldn’t let it be called KYW-TV (at least it wasn’t called that) and used WPTZ. Now I have seen that WPTZ is a Plattsburgh station (how reasonable) and the original KYW TV station is now called–surprise–KYW-TV. Will wonders never cease?
You have a very interesting site! If you care, an addition to your listing you might consider: WLPI-KLPI Ruston, Louisiana. I was a DJ there in the early '70s while a student at Louisiana Tech. WLPI-AM went off air in '74, but KLPI-FM carries on.
BTW: I DXed a lot when I was a kid, and Whites Radio Log was my Bible … Any Info on what became of it would be greatly appreciated.