A few further corrections:
CW said:
ALL Radio callsigns are internationally agreed upon. The US has AA-AZ, WA-WZ, KA-KZ, K, W, N, NA-NZ and I think one other prefix assigned to it. Canada has C, VE and Mexico has X, XE, and a couple of other prefixes. US amateur radio stations use all these prefixes in various versions, planes are assigned Nxxxx calls, and radio stations were originally assigned Wxxx and Kxxx calls..either 3 or 4 letter calls.
The US blocks are AAA-ALZ, KAA-KZZ, NAA-NZZ, WAA-WZZ. (AMA-AZZ is split among several countries - AM to Spain, AX to Australia, AZ to Argentina, among others)
The blocks are established in groups of 3 letters. You can drop one or more if they're unique to your country. (so we can issue "W9WI" to an amateur station because the entire WAA-WAA block belongs to the USA. However, we can't issue "A1AW" because other countries get some of the AAA block. AA1AW would be valid since we have all of AAA-AAZ.) The only countries that got entire first letters to themselves: China (B); France (F); U.K. (G, M, 2); Italy (I); USA (K, N, W); and Russia (R).
Canada and Mexico do
not have all of the CAA-CZZ and XAA-XZZ blocks. CXYA would be a Uruguayan callsign and XZXZ is Burman. (/Myanmaran, if you prefer) Actually, Canada got some of the X block - XL2EBM would be a Canadian amateur callsign. Canada uses only CFA-CKZ (except CGA-CGZ) for broadcasters -- and CBA-CBZ which isn't theirs! (it belongs to Chile) Mexico uses XEA-XEZ and XHA-XHZ, the latter only for FM and TV stations.
FCC policy is to only use the K--- and W--- blocks for broadcast stations. (there was some discussion a few years ago about opening the A and N blocks but not much interest among broadcasters, strangely enough. Likewise for a proposal to drop the K/W distinction and allow W calls in the West and vice-versa.)
In the 1940, the 3 letter calls were dropped and only 4 letter calls were given (The older stations still have 3 letter calls like WRR, WLS, KOA, KEX, etc.)
The last new 3-letter call was issued to WIS (now WVOC) in Columbia, South Carolina in 1930. Since then, a literal handful (OK, maybe two handfuls) of 3-letter calls have been reissued to stations which once held them, or to stations with some kind of relationship to a station which once held them.