ITU prefix

The International Telecommunication Union (ITU) allocates call sign prefixes for radio and television stations of all types.^[1] They also form the basis for, but may not exactly match, aircraft registration identifiers. These prefixes are agreed upon internationally, and are a form of country code. A call sign can be any number of letters and numerals but each country must only use call signs that begin with the characters allocated for use in that country.

With regard to the second and/or third letters in the prefixes in the list below, if the country in question is allocated all callsigns with A to Z in that position, then that country can also use call signs with the digits 0 to 9 in that position. For example, the United States is assigned KA–KZ, and therefore can also use prefixes like K1 or K9.

While ITU prefix rules are adhered to in the context of international broadcasting, including amateur radio, it is rarer for countries to assign broadcast call signs to conventional AM, FM, and television stations with purely domestic reach; the United States, Canada, Mexico, Japan, South Korea, the Philippines, and Argentina are among those that do. Canada presents one notable exception to the ITU prefix rules: Since 1936, it has used CB for its own Canadian Broadcasting Corporation stations, whereas Chile is officially assigned the CB prefix. Innovation, Science and Economic Development Canada's broadcasting rules indicate this is through a "special arrangement", without elaborating.^[2] In any case, the two countries are geographically separate enough to prevent confusion; Canada's shortwave broadcasters and amateur radio stations have always used one of its assigned ITU prefixes.