Many countries have specific conventions for classifying call signs by transmitter characteristics and location. The North American call sign format for radio and television call signs follows a number of conventions. All call signs begin with a "prefix" assigned by the International Telecommunications Union. For example, the United States has been assigned the following prefixes: AAA-ALZ, K, N, W. For a complete list, see International Callsign Allocations.

Canadian broadcast stations are assigned a three-, four- or five-letter callsign beginning with CF, CH, CI, CJ, CK, VE, VF, or VO. Several other codes are available, but are not currently in use. Stations of the Canadian Broadcasting Corporation or Société Radio-Canada tend to identify themselves as "CBC Radio One/Two" (English-language AM/FM) or "La Première Chaîne/La Chaîne culturelle" (French-language AM/FM) of a city, although they do have official three- and four- letter callsigns. These generally begin with CB by arrangement between the Canadian and Chilean authorities. Callsigns beginning with VE and VF are only assigned to very-low-power local rebroadcasters; VO callsigns may only be used by four stations in Newfoundland which were licensed before that province joined Canadian Confederation in 1949. All Canadian FM stations have a "-FM" suffix, and likewise TV stations and "-TV", except for low-power rebroadcasters which have seminumeric callsigns. Higher-power rebroadcasters are generally licensed under the callsign of the originating station, followed by a numeric suffix and, for FM rebroadcasters of an AM station, a "-FM" suffix; for example, CJBC-1-FM rebroadcasts CJBC (860 Toronto), whereas CJBC-FM-1 rebroadcasts CJBC-FM (90.3 Toronto).

Canadian stations are required to identify by callsign hourly, but not at any specific time, and this rule is even more rarely enforced than the U.S. rule (see below).

Mexican broadcast stations are assigned a three-, four-, five-, or six-letter callsign beginning with XE (mediumwave) or XH (all others). Some FM and TV stations are grandfathered with XE callsigns and a "-FM" or "-TV" suffix. Mexican stations are required to identify twice an hour, at both the top and the bottom. Mexican stations broadcasting English-language programming are in addition required to play the Mexican national anthem every day at midnight local time. As in Canada, stations that rebroadcast other stations have the same callsign, but with a different number at the end (such as XEMN and XEMN-1).

In the United States, broadcast stations have callsigns between three and six characters in length, though the minimum length for new stations is four letters. Full-power stations receive four-letter callsigns, while broadcast translators and low-power stations usually receive callsigns with five or six characters, including two or three numbers. Stations with three letters were granted those names in the 1920s and have been grandfathered into the current system, even though many such stations have changed owners. These stations include KOA in Denver, Colorado, WGN in Chicago, Illinois, and WRR in Dallas, Texas. (WRR is an unusual case in that the callsign was moved from the original AM station to a commonly-owned FM station, formerly WRR-FM, before the AM was sold.) The Federal Communications Commission for many years maintained a policy of "drop it and lose it forever" with respect to three-letter callsigns, but recently allowed KKHJ (930 Los Angeles) to regain its historic three-letter call, KHJ.

New stations are assigned a code beginning with K, if they are west of the Mississippi River, and beginning with W if they are east of the river. Again, some early stations have been grandfathered in, so there are four broadcasters with a K prefix east of the Mississippi, and a few dozen with a W on the west side. (There are more grandfathered Ws because the dividing line used to be one state farther west.) Some examples would be KDKA in Pittsburgh, Pennsylvania, KYW in Philadelphia, and WDAY in Fargo, North Dakota. Stations located near the Mississippi River may have either letter, depending on the precise location of their community of license and on historical contingencies. Metro areas with mixed W- and K- stations because of proximity to the river include Minneapolis-St. Paul, Minnesota, St. Louis, Missouri, and New Orleans, Louisiana.

FM and TV translators, and many (but not all) low-power TV stations are assigned sequential callsigns. They use an appropriate initial letter followed by a two- or three-digit channel number, and then a two-letter sequential suffix. For example, a TV translator on channel 4 might have the call sign K04AX (though it is much less common for TV translator channels to be between 2 and 13). The FM band also has channel numbers starting at the number 200 (or 201 for practical purposes), although they are almost unknown to regular listeners who usually tune in to a station based on its frequency. W201AA was the first FM translator at 88.1 MHz, for example. Such callsigns are never reused by another station.

New full-power stations are assigned sequential callsigns if the permittee does not choose one of their own; these are always four letters, of which the third is the least-significant digit and the second is the most-significant digit of the sequence number. (Callsigns which were already assigned are skipped in the sequence.) Hence, many very early stations, like WMAQ Chicago (now WSCR) and WMAF South Dartmouth (now defunct) were assigned W-A- or K-A-) callsigns.

FM radio and television callsigns may be followed by a dash and the two-letter class of station: "-FM", "-LP", "-TV", or "-CA". For digital television, the "-HD" and later "-DT" suffixes are no longer officially used, as the digital channel is not licensed separately from the analogue. (Some radio owners using the iBiquity "HD Radio" IBOC system have expressed a desire for "-HD" callsigns, but this is unlikely to happen for the same reason.) Occasionally, an FM or TV station may have "boosters", which are similar to translators but operates on the same channel. In this case, the main portion of the callsign remains the same (unlike with translators), and the boosters are given sequential numeric suffixes like -FM1, -TV2, -3, and so forth.

Low-power LPTV and LPFM stations share the -LP suffix. Those LPTV stations protected from interference by so-called "primary" stations use the -CA suffix. Instead of a suffix, translator/repeater stations get a W or K, the channel number (2~69 for TV, 201~300 for FM), and two serial letters, such as W02AA, or K201AA (88.1).

Many stations prefer not to use callsigns at all, since a slogan is more easily remembered by listeners (and those filling in diaries for the Arbitron radio ratings). However, in the U.S., the Federal Communications Commission does require periodic identification using the formal callsign, as close to the top of each hour as possible, at a "natural break in programming"; though this rule is rarely enforced. Radio stations are also required to identify their community of license. There are some unusual cases, though, such as the low-frequency WWVB time station. That station broadcasts a one bit per second signal that cannot be understood by humans, so the station is identified by shifting the broadcast carrier wave's phase by 45° (see PSK).

It is fairly common for stations to choose a callsign that can be transformed into a name, such as Boston's WXKS-FM (107.9 Medford), one of many Clear Channel Communications-owned stations that call themselves "KISS". In other instances, the letters may be an initialism for a name or slogan. Some of the most famous of these include WGN, owned by the Chicago Tribune, which stands for "World's Greatest Newspaper", and WLS, then owned by Sears Roebuck, the "World's Largest Store".

Extremely early callsigns used in the 1910s and into the early 1920s were arbitrary. The U.S. government began requiring stations to use three-letter callsigns around 1912, but they could be chosen at random. KDKA initially broadcast as 8XK before gaining its well-known letters in 1920. The Rosicrucian Order, AMORC of San Jose, California used the callsign 6KZ.

Experimental broadcast stations (indeed, all experimental stations, broadcast or not) use callsigns out of the amateur radio sequence, with the letter following the region digit required to be an X. (All VHF stations before World War II were licensed as experimental stations.) Notable experimental stations included Major Armstrong's FM station W2XMN in Alpine, New Jersey; Powell Crosley's 500-kW superpower AM W8XO, operating nights only with WLW's programming and frequency from Mason, Ohio; and Don Lee's pioneering television station, W6XAO in Los Angeles. (Synchronous "booster" transmitters for AM stations are still considered experimental in the U.S., despite fifty years of experience in Europe, and new experimental callsigns are being assigned for new licenses even now, by inserting a region digit and the letter X into the parent station's callsign.)

Somewhat confusingly, the National Weather Service also uses similar-looking callsigns for weatheradio stations (like KEH79), as does the ICAO airport code system (like KNYC).