All call signs begin with a "prefix": letters and number assigned by the International Telecommunications Union. The United States has been assigned the following prefixes: AAA-ALZ, K, N, W. For a complete list, see International Callsign Allocations.

Many countries have specific conventions for classifying call signs by transmitter characteristics. The North American call sign format for radio and television call signs follows a number of conventions.

In the United States, the vast majority of stations east of the Mississippi River have call signs beginning with "W". Exceptions include: KDKA (Pittsburgh), the first commercial radio station in the world, and KYW (Philadephia). Callsigns of US stations west of the Mississippi River generally begin with "K". Among the grandfathereded exceptions to this are WDAF (Kansas City), WTAW (Bryan, Texas), and WACO (Waco, Texas), all issued before the rule took effect.

Many stations also get suffixes to identify which band they are on, and to create separate callsign pools for each type of service. AM, FM, and TV naturally get -AM, -FM, and -TV tacked on. Digital TV stations started with the -HD suffix, but now get -DT instead, and always have the base callsign of the analog station (i.e. WABC-TV's digital station must be WABC-DT).

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).

In Canada, 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 usually begin with "CB", which actually is assigned to Chile internationally, but is "borrowed" for domestic broadcast use only. Non-CBC stations use a four-letter callsign (two exceptions being CKX and CKY in Manitoba) beginning with "CF" through "CK" -- though older stations in Newfoundland, like VOCM St. John's, usually use "VO" (the ITU prefix assigned before it officially joined the Dominion of Canada).

In Mexico, stations usually have four or five letters in a callsign, but sometimes few as three or as many as six, such as XHMORE in Tijuana. FM stations begin with XH, and AM ones with XE. 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).