A group of letters and sometimes numbers used to identify a television or radio station. To account for the numbers, the term "callsign" is actually more correct, but in the United States, the callsigns given to full power broadcasters contain only letters, so the "call letters" term has stuck.

The International Telecommunications Union is responsible for assigning various blocks of letters and/or numbers to all the countries on Earth for use in callsigns. For example, Nauru is the only country that can use call letters beginning with C2; France gets to use FA through FZ, HWA through HYZ, TH, TK, TM, TOA through TQZ, and TVA through TXZ. (Many of the other countries holding a portion of the T's are former French colonies in Africa.) Beyond the initial letters or numbers, it's up to each individual government how they want to divvy up callsigns.

Very few countries require broadcasters to use their callsigns on the air frequently; in many cases, it's only at sign on and signoff, and sometimes not even then. However, the Federal Communications Commission in the U.S. requires broadcasters to announce their call letters and city of license once an hour, and as such, radio and TV stations there tend to be known more by their call letters than anything else.

The FCC's general scheme for assigning radio and TV call letters is that stations east of the Mississippi River have call letters beginning with W, and stations west of the Mississippi have call letters beginning with K. Some stations violate this rule because they began broadcasting before the FCC had the rule in place. One of the best-known examples is KDKA in Pittsburgh, one of the first commercial radio stations.

After the first letter, the next three letters in the sequence can be chosen by the licensee, although they were assigned in sequence in the early days (again, KDKA is an example of this). Some older stations have only two letters after the W or K, such as WBZ in Boston, KHJ in Los Angeles, and another Pittsburgh radio station that's breaking two current rules, KQV.

In most cases, the person doing the call letter picking tries to come up with a memorable set, since the station is going to have to use them to identify itself anyway. There are plenty of ways to do this:

If there is more than one station with the same first four (or three) call letters, a 2-letter suffix is added to indicate what type of station it is:

For example, in New York, 880 kHz is WCBS, 101.1 MHz is WCBSFM, Channel 2 is WCBSTV, and Channel 56 is WCBSDT, as far as the FCC is concerned. Everyone else, though, would render those with a hyphen, i.e. WCBS-FM, WCBS-TV, and WCBS-DT.

At least since the 1970s, many American radio stations have tried to deemphasize their call letters, instead primarily identifying themselves by a different name, usually including the frequency. However, many of these will either be based on the existing call letters anyway, or will result in a call letter change when the name changes. There are a ton of radio stations calling themselves "Kiss FM," for example, but many have call letters such as KIIS or WVKS.

The same thing has been happening in television since the late 1980s, when the Fox network persuaded many of their affiliates to refer to themselves as "Fox (channel number)." This idea has since been copied by the other networks and their affiliates.

However, not only are the FCC regulations still in effect requiring stations to identify themselves by their call letters once an hour, more importantly, both the Nielsen television ratings and the Arbitron radio ratings primarily use the call letters to determine who was watching or listening to what. If a misguided viewer in Los Angeles writes in a Nielsen diary that he was watching "7 News" on NBC 4, KCBS, the ratings will go to Channel 2, KCBS. More than anything else, that assures the future of call letters in the U.S.