|The Geneva-based International Telecommunication Union (or ITU) is the worldwide governing body over wireline and radio communications. A specialized agency of the United Nations, the ITU is made up of nearly 200 countries who agree on telecommunications matters.
As an aid to enforcement of the international radio laws and regulations, most transmitting stations of the world are required to identify themselves at regular intervals when they are in operation. Each station is assigned a call sign, a combination of letters or letters and numbers that identifies that station.
By international agreement, the first characters of the call sign indicate the country in which the station is authorized to operate. Therefore, anyone monitoring the airwaves is able to identify the national origin of a radio transmission by its station call sign prefix.
Although the U.S. amateur service only uses a maximum of two letter prefixes, up to three prefix numbers or letters are allocated by the ITU. The national prefixes assigned to the United States and its possessions are: AAA-ALZ*, KAA-KZZ, NAA-NZZ and WAA-WZZ. No other prefixes are authorized.
(* The balance of the "A" prefixes: AMA-AOZ is assigned by the ITU to Spain, APA-ASZ to Pakistan, ATA-AWZ to India, AXA-AXZ to Australia, and AYA-AZZ to Argentina.)