In the United States, all radio and television broadcasting stations that are licensed by the Federal Communications Commission are assigned official, and unique, call signs. Organized broadcasting began in the U.S. in the early 1920s on the AM band — FM and television did not exist yet. Initially most broadcasting stations were assigned three-letter calls; however, a switch was made in April 1922 to primarily four-letter calls, after the number of stations had increased into the hundreds. For a few years thereafter a small number of new three-letter calls continued to be issued. Although most of the original three-letter calls were randomly assigned, these later calls were often specially requested to match station slogans. The last new three-letter call was assigned to station WIS in Columbia, South Carolina on January 23, 1930. Since then, three-letter calls have only been assigned to stations, including FM and TV, which are historically related to an AM station that was originally issued that call sign. This review only includes FCC-licensed stations. Not included are unlicensed operations, such as carrier current, cable TV, and Internet stations — for example, San Diego State University's "KCR" — which have adopted call-letter-like identifiers that are not officially issued by the FCC. Also not included are stations which use, as slogans, three-letter truncations of their official four-letter call signs; for example, the full call sign for radio station"KOH" in Reno, Nevada is actually KKOH. In addition, stations which formerly had three letters but have since changed are not listed. there are a total of 103 AM, FM and TV stations in the United States that are assigned three-letter call signs. This is divided between only 68 different three-letter calls, because in many cases the same call sign is used by more than one station, although a given call sign is never assigned to more than one AM, FM or TV station. These 68 different three-letter call signs are currently grouped as follows:
In cases where an AM station exists with the same "base" call letters as an FM station, the FM station is required to include an "-FM" suffix as part of its call sign. If no AM station is currently using the same base call sign, then the "-FM" suffix is optional.
Television (21 stations)
As with FM stations, if an AM station currently exists with the same "base" call, a TV station must include a suffix to differentiate itself, but this suffix is optional if no AM station currently exists. TV stations may select either "-TV" or "-DT" as their suffix.
