Federal Communications Commission (FCC) regulations require that each TV station identify itself at least once each hour by call letters and by city of license. The announcement should be made at or close to the hour during a natural break in programming and can be made either visually or aurally. Stations have the option to insert their channel numbers between the call letters and the city of license, and virtually all stations follow this practice: for example, KRON-TV, channel 4, San Francisco. In advertising and promotional announcements, stations generally promote their channel assignments more vigorously than their call letters.

Some of the more ingenious call letters actually identify the channel either by word or by Roman numeral. These include KTWO, Casper, Wyoming; KFOR, Oklahoma City; WTEN, Albany, New York; and KTEN, Ada, Oklahoma. Two Roman numeral examples include WIXT, Syracuse, New York, and KXII, Ardmore, Oklahoma. Two other stations, WPVI, Philadelphia, and KPVI, Pocatello, Idaho, both use a P for their respective cities followed by Roman numerals to indicate their channel-6 assignments.

The procedures for assigning call letters have their origin in the earliest days of radio. Blocks of initial letters were assigned to various countries following the London International Radiotelegraph Conference of 1912. The letters W, K, N, and A were assigned to the United States. W and K were used to designate commercial broadcasters, whereas N and A were allocated to military users of the radio spectrum. The initial letters C and X were assigned to Canada and Mexico, respectively, and are still used today to identify Canadian and Mexican television stations.

The first U.S. radio stations were allowed to select their own call letters beginning with either a W or a K. Also, early radio stations could select either a three-letter or a four-letter combination. Later, around 1928, the Federal Radio Commission formalized rules requiring that all stations use four-letter combinations. Further, those stations east of the Mississippi were required to use an initial W, while those stations west of the Mississippi were required to use an initial K.

Stations already on the air were allowed to keep their call letters regardless of number or location. Radio and, later, television stations such as KDKA, Pittsburgh, Pennsylvania; WGN, Chicago; WHO, Des Moines, Iowa; and WOW, Omaha, Nebraska, demonstrate their pioneer status and their unbroken ownership by being notable exceptions to the current rules. When WOR-TV, New York, was acquired by a new owner, the station was required to adhere to the four-letter requirement and became WWOR-TV.

Call letters often tell something about station ownership. New York stations WABC-TV, WCBS-TV, and WNBC-TV are each owned and operated by the respective networks contained within their call letters. So too are Los Angeles stations KABC-TV, KCBS-TV, and KNBC-TV. Ted Turner’s WTBS (Turner Broadcasting System) is still another example. A change in ownership will often, but not always, bring a change in call letters. When Philadelphia TV station WTAF was sold by Taft Broadcasting to another owner, it became WTXF.

Some TV call letters trace their origins to the slogans of their radio station predecessors. Examples include WGN (World’s Greatest Newspaper), the Chicago station owned by the Chicago Tribune; WLS (World’s Largest Store), the Chicago station originally owned by Sears Roebuck; WSM (We Shelter Millions), the Nashville, Tennessee, station originally owned by an insurance company; and WSB (Welcome South, Brother), the Atlanta, Georgia, station that conveys regional boosterism in its call letters.

Public television stations have continued this tradition. Chicago’s WTTW (Windows to the World) and Philadelphia’s WHYY (Wider Horizons for You and Yours) are two examples. Both WQED, Pittsburgh, and KQED, San Francisco, use the abbreviation for the Latin phrase quod erat demonstrandum (which was to be proven) in their call letters.

The growth of cable has increased the promotional value of call letters since some cable systems retransmit TV signals “off-channel.” For example, a VHF station that broadcasts on channel 10 might be carried on cable channel 5; a UHF station that broadcasts on channel 48 might be carried on cable channel 13. As a result, many TV stations continue to identify themselves by channel assignments but also promote their call letters more extensively than in the past.

When television stations broadcast a digital signal in addition to the traditional analog signal, they usually add DT to the call letters for that signal. Those stations broadcasting a high-definition signal may add an HD to their call letters.