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, VF, VO, or VX. 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 with four digits preceded by VF (for radio) or CH (for television) 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).

Canadian amateur radio stations generally begin with VE, although some use VA. The number following these letters indicates the province, going from 1 for Nova Scotia, through 7 for British Columbia, to 8 for the Northwest Territories, with latecomer VE9 for New Brunswick (VE1 used to be for all 3 Maritime Provinces). VE0 is for maritime mobile. VY1 is used for the Yukon Territory, VY2 for Prince Edward Island, and VY0 for Nunavut. CY0 is used for Sable Island and CY9 for St. Paul Island. Special prefixes are often issued for stations operating at significant events.

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).

Amateur radio stations in Mexico use XE1 for the central region, XE2 for the northern, and XE3 for the southern. XF prefixes indicate islands. Special callsigns for contests or celebrations are occasionally issued.

In the United States, broadcast stations have call signs between three and six characters in length, though the minimum length for new stations is four letters. An additional suffix may also be added, indicating a specific broadcast sevice type. Full-power stations receive four-letter call signs, while broadcast translators and low-power stations usually receive call signs with five or six characters, including two or three numbers. Generally, station names begin with "K" west of the Mississippi River, and "W" to the east.

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 call sign 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 call signs, but recently allowed KKHJ (930 Los Angeles) to regain its historic three-letter call, KHJ.

The FCC allows FM and TV stations under common ownership with a three-letter AM or FM in the same market to use five-letter (three plus "-FM" or "-TV" suffix) callsigns; e.g., KGO-TV in San Francisco or WMC-FM in Memphis. In some cases, such as WIL-FM in St. Louis, the five-letter callsign may outlive the three-letter callsign on which it is based. There is also the unusual case of Baltimore's WJZ-TV, which was allowed to adopt the callsign despite the fact that there was no longer a WJZ radio, it wasn't in Baltimore, and it hadn't been owned by the same company.

Extremely early call signs used in the 1910s and into the early 1920s were arbitrary. The U.S. government began requiring stations to use three-letter call signs 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 call sign 6KZ.

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 "W" 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, St. Louis, New Orleans, and the Quad Cities. Notable significant exceptions to this rule are the time signal stations WWV and WWVH, in Colorado and Hawaii respectively.

FM radio and television call signs 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" call signs, but this is unlikely to happen because HD Radio is a subcarrier service on the same frequency.) 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 call sign remains the same (unlike with translators), and the boosters are given sequential numeric suffixes like -FM1, -FM2, -3, and so forth. Only FM boosters use this sequence, as AM boosters have a different numbering scheme, and TV boosters are not practical with NTSC or ATSC broadcasts, and are therefore not allowed.

Low-power LPTV and LPFM stations share the -LP suffix. Those LPTV stations protected from interference by primary stations use the -CA suffix.

FM and TV translators, and many (but not all) low-power TV stations are assigned sequential call signs. 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 call signs are never reused by another station.

New full-power stations are assigned sequential call signs 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-) call signs.

Many stations prefer not to use call signs 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 call sign, 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. Because of the station's narrow signal, that station only broadcasts a one bit per second signal that cannot usually be understood by humans, so the station is identified by shifting the broadcast carrier wave's phase by 45° twice an hour. (see PSK).

It is fairly common for stations to choose a call sign 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".

Experimental broadcast stations (indeed, all experimental stations, broadcast or not) use call signs 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 call signs are being assigned for new licenses even now, by inserting a region digit and the letter X into the parent station's call sign.)

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

In the United States amateur radio callsigns are issued with 1 or 2 letters, followed by a signal digit number, and then 1 to 3 more letters. Generally the shorter the call (up to a 1x2 or 2x1 format) the higher the license, but an amateur who upgrades is not required to change his or her callsign. In any case some of the available blocks have been used up. The 1x1 callsigns, such as K6O, are for short-term special event stations. Outlying areas have special calls. For example, those issued in Hawaii can (like other U.S. callsigns) start with A, K, N, or W, but then will have "H6" before the 1-3 additional letters. Other Pacific possessions use other "H" numbers; a Guam station could be KH0. Alaska has "L" as the second prefix letter, and Caribbean stations use "P."

The number in the call refers to one of the 10 radio districts into which the U.S. is divided, but that onlly indicates where the license was issued. Unlike many countries it is no longer necessary for a U.S. ham to change callsigns when moving to a new district. Most amateurs going to an exotic location will sign /(prefix) to show their location. Thus a station visiting American Samoa could be (regular call)/KH8.