How the US got its four prefix blocks — A, K, N, and W — across 35 years and four conferences. "A for Army, N for Navy" turns out to be only half as old as it sounds.
The US holds four prefix blocks and no others. Everything below is the story of how each one arrived.
Sources: ITU Appendix 42 (Rev. WRC-23); FCC, "Amateur Call Sign Systems"; Wikipedia, "Call signs in the United States."
The London Radiotelegraph Convention assigned the first national prefix blocks. The US received N, W, and KDA–KZZ — the tail end of K. Germany and its protectorates got A, D, and KAA–KCZ — the front of K.
So K wasn't shared; it was sliced. Germany held everything up to KCZ, the US everything from KD onward. In 1912 the letter A did not belong to the United States at all.
Sources: International Radiotelegraph Convention (1912) record; broadcasting-history archive (Call sign / US call letter assignments); primary Department of Commerce call-letter description (earlyradiohistory.us).
Inside the US, combinations beginning with N were reserved for government stations. The Army of the United States was assigned specific W sub-blocks — WUA–WVZ and WXA–WZZ — not A. K and W were otherwise divided by coast for ship and land stations, the arrangement that later flipped into the familiar broadcast "K west of the Mississippi, W east."
The correction worth banking: the popular "A for Army" is not the original scheme. When the Army first had reserved calls, they were W calls.
Sources: Period Department of Commerce call-letter description, reproduced at earlyradiohistory.us ("United States Callsign Policies").
From October 1, 1928, W and K prefixes were added to US amateur calls to make them internationally unique. A ham who had been 8ABC became W8ABC — the prefix simply bolted onto the front of the old call.
Sources: earlyradiohistory.us (US callsign policy); Ham Radio Prep, "Ham Radio Call Signs."
The KA–KC slice that had been Germany's since 1912 was finally assigned to the United States, giving it the whole K block for the first time.
Sources: earlyradiohistory.us (note: KAA–KCZ to the US in 1929); broadcasting-history archive (US call letter assignments).
At the Atlantic City ITU conference, the US was recorded as receiving the AA–AL blocks — the reallocation that locked in the modern A/K/N/W shape. The same conference reshuffled US call districts: a 10th district was formed for the central Midwest, and the 6th was narrowed to California only.
Two domestic redraws at the same conference. Each state is colored and labeled by its call district; the states that changed districts are outlined in dark gray.
The old 9th district was split, opening a new 0 district across the central plains and leaving Illinois, Indiana, and Wisconsin to the 9th. In the west, the 6th contracted to California alone, and Nevada, Arizona, and Utah moved into the 7th.
Sources: Ritz (W7VO) callsign history; Ham Radio Prep and W2ZQ (1927 = K/N/W only); Wikipedia regional callsign articles (1947 Atlantic City reallocation).
The US now holds AAA–ALZ, KAA–KZZ, NAA–NZZ, and WAA–WZZ. "N for Navy" is the genuinely old half of the mnemonic. "A for Army" is a post-1947 domestic overlay — it only became possible once the US actually held the A block.
Sources: FCC, "Amateur Call Sign Systems"; Wikipedia, "Call signs in the United States."
One origin story holds that the civilian prefixes were built from the military ones by adding a single Morse dash: A (·−) plus a dash becomes W (·−−); N (−·) plus a dash becomes K (−·−). Memorable, but it rests on fewer independent sources than the dates above, so treat it as folklore worth telling, not established fact.
Sources: broadcasting-history archive (Call sign).