Call signs in the United States are identifiers assigned to radio and television stations, which are issued by the Federal Communications Commission (FCC) and, in the case of most government stations, the National Telecommunications and Information Administration (NTIA). They consist of from 3 to 9 letters and digits, with their composition determined by a station's service category. By international agreement, all call signs starting with the letters K, N, and W, as well as AAA-ALZ, are reserved exclusively for use in the United States.
AM, FM, TV and shortwave broadcasting stations can request their own call letters, as long as they are unique. The FCC policy covering broadcasting stations limits them to call signs that start with a "K" or a "W", with "K" call signs generally reserved for stations west of the Mississippi River, and "W" limited to stations east of the river. Amateur stations can receive call signs starting with all of the letters "A", "K", "N", and "W". Formerly, prefixes beginning with "A" were exclusively assigned to U.S. Army stations and prefixes beginning with "N" to U.S. Navy stations.