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 (not including the -FM or -TV suffix) beginning with CB, CF, CH, CI, CJ, CK, VA–VG, VF, VO, VX, or VY. The CB series calls are assigned to Chile by the ITU, but Canada makes de facto use of this series anyway for stations belonging to, but not exclusively broadcasting programs from, the Canadian Broadcasting Corporation [1]. Several other codes are available, but are not currently in use. Conventional radio and television stations almost exclusively use the C codes; with a few exceptions noted below, the V codes are restricted to specialized uses such as amateur radio.

Special broadcast undertakings such as Internet radio, cable FM or closed circuit stations may sometimes be known by unofficial call signs such as CSCR. These are not governed by the Canadian media regulation system, and may at times reflect call signs that would not be permissible on a conventional broadcast platform.

Four-letter callsigns are the norm. Three-letter callsigns are only permitted to CBC Radio stations or to commercial stations which already had a three-letter callsign before the current rules were adopted, and five-letter callsigns exclusively identify CBC transmitters (which may be either rebroadcasters or SRC O&Os outside of Quebec.)

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/Espace Musique" (French-language AM/FM) of a city, although they do have official three- and four- letter callsigns. These generally (but not always) begin with CB.

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 commercially by stations in Newfoundland and Labrador which were licensed before that province joined Canadian Confederation in 1949 (VOCM, VOAR and VOWR broadcast from St. John's long before Confederation). Only one station, VOCM-FM, has been allowed to adopt a VO callsign after 1949; it was granted the VOCM calls because of its corporate association with the AM station.

All Canadian FM stations have an "-FM" suffix, 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). Some rebroadcasters, however, may have their own distinct callsigns.

For rebroadcasters which use a numeric suffix, the suffixes usually follow a 1-2-3 numeric sequence which indicates the chronological order in which rebroadcast transmitters were added. There are some cases where television rebroadcasters are suffixed with the channel number on which the transmitter broadcasts (for instance, CIII-TV's rebroadcasters are numbered with their channel assignment rather than sequentially), but this is not generally the norm.

Canadian broadcast 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 VA1/VE1 for Nova Scotia, VA2/VE2 (Québec), VE3/VA3 (Ontario) through VA7/VE7 for British Columbia and VE8 for the Northwest Territories, with latecomer VE9 for New Brunswick (VE1 used to be for all three Maritime Provinces). VE0 is for maritime mobile amateur transmissions. 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.

The Dominion of Newfoundland prefix VO remains in active use by radioamateurs in the province of Newfoundland and Labrador, VO1AA atop Signal Hill in St. Johns being the most famous amateur station. Radio amateurs on the Island of Newfoundland use calls beginning with VO1, while Labrador amateurs use VO2. A popular backronym for VO stations is "Voice Of...", although this is not the VO prefix's official meaning.

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.

The islands of the Bahamas and Bermuda have the call signs of VS and Z, pertaining to their status as former (and in the case of Bermuda, current) colonies of England, sharing the UK and other colonies' reserved call letter of Z.

All of the former British West Indies colonies share the VS, ZB-ZJ, ZN-ZO, and ZQ prefixes. The list is as shown:

• Anguilla
• Antigua and Barbuda
• Bahamas (Also has the C6 Prefix)
• Barbados
• Bermuda (Also uses VS)
• British Virgin Islands
• Cayman Islands
• Dominica (Commonwealth of Dominica, also uses J7)
• Grenada (also uses J3)
• Jamaica
• Montserrat
• St. Kitts and Nevis
• St. Lucia (Also uses J6)
• St. Vincent and the Grenadines (Also uses J8)
• Turks and Caicos Islands

Cuba uses the prefixes CL-CM, CO, and T4.

The Kingdom of the Netherlands use the PA-PI prefixes, while the Netherlands Antilles use the PJ prefix.

The island nation of Trinidad and Tobago use the 9Y-9Z prefixes.

Puerto Rico, Navassa Island, and the US Virgin Islands all use the American standard call signs of W (being east of the Mississippi River).

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 service type. Full-power stations receive four-letter call signs, while broadcast translator stations usually receive call signs with five or six characters, including two or three numbers. Generally, call signs begin with "K" west of the Mississippi River, and "W" to the east.

In the 1920s, many stations were assigned three-letter call signs; these have been grandfathered under the current system, even though many such stations have changed owners. Such 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; for example, 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; when there was, it wasn't in Baltimore; and it hadn't been owned by the same company since the 1920s.

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 broadcasting 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, 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" stations 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, Memphis, 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. (WWV originally began in Maryland and was later moved west. However, even ignoring that fact, U.S. government-owned stations are overseen by the NTIA and not the FCC, and are thus not subject to the FCC's rules on call signs—most do not have call signs at all.)

The FCC allows derived callsigns in the same market as a commonly-owned AM or FM without respect of the boundary, so stations may establish common branding across bands and services. One famous example was the case of the former KWK in St. Louis, which after several petitions was permitted to change the callsign of its sister FM station in Granite City, Illinois, then WWWK(FM), to KWK-FM. Later, the AM would change its callsign and the FM became KWK(FM), thereby becoming an exemplar of both categories of grandfathered stations.

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 operate on the same channel. In this case, the main portion of the call sign remains the same (unlike 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. Some translators and boosters even have the "-TX" suffix (Translator), which is reserved for radio and television stations, such as W48AV, but it is rarely, if ever, shown Those LPTV stations protected from interference by primary stations use the "-CA" suffix. Upon completion of the conversion to Digital Television, LPTV stations will gain the "-LD" (Low-Power Digital) and "-DL" (Digital Low-Power) suffixes, and "-CA" will get "-DC" (Digital Class-A) suffix.

FM and TV translator 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 Round Hill, 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 Phase-shift keying).

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," WCFL, which stands for "Chicago Federation of Labor", and WLS, then owned by Sears Roebuck, the "World's Largest Store."

United States amateur radio callsigns are issued with one or two letters, followed by a single digit, and then one to three more letters. Generally the shorter the call (up to a 1x2 or 2x1 format) the higher the grade of 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 only indicates where the license was issued. 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. American amateurs are also permitted operate in Canada under their own callsigns with a location indicator.

Experimental stations 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, as does the ICAO airport code system.