United States
The earliest identification, used in the 1910s and into the early 1920s, was arbitrary. The U.S. government began requiring stations to use three-letter call signs around 1912, but they could be chosen at random. This system was replaced by the basic form of the current system in the early 1920s. Examples of pre-1920 stations include 8XK in Pennsylvania, which became KDKA in November 1920 and remained since; and Charles Herrold's series of identifiers from 1909 on in San Jose, California: first "This is the Herrold Station" or "San Jose calling", then the call signs FN, SJN, 6XF, and 6XE, then, with the advent of modern call signs, KQW in December 1921, and eventually KCBS from 1949 on.
All broadcast call signs in the United States begin with either "K" or "W", with "K" usually west of the Mississippi River and "W" usually east of it (except in Louisiana and Minnesota, which don't strictly follow the dividing line between the two groups). Initial letters "AA" through "AL", as well as "N", are internationally allocated to the United States but are not used for broadcast stations.
In the United States, broadcast stations have call signs of three to seven characters in length, including suffixes for certain types of service, but the minimum length for new stations is four characters, and seven-character call signs result only from rare combinations of suffixes.
Each station with a traditional full-power license, whether AM, FM, TV, or private shortwave, has a call sign of three or four letters, plus an optional suffix of either "-FM" or "-TV". A broadcast translators or other low-power station has either four letters with a mandatory two-letter suffix indicating its type, or a five or six character call sign consisting of "K" or "W", followed by two to three digits indicating its frequency, followed by two letters issued sequentially.
For most of the 20th century, new full-power stations were also assigned their four-letter call signs sequentially by the Federal Communications Commission (FCC) if the permit holder did not choose a call sign prior to full licensing. For four-letter call signs beginning with W, sequential call signs were incremented with the fourth letter being least significant and changing each time, the second letter being the next most significant, and the third letter being the least significant and changing only after every combination of second and fourth letter was exhausted. Thus, the first sequential four-letter call signs in the W sequence were of the W_A_ form, such as WMAF South Dartmouth, Massachusetts (now defunct) followed soon after by WMAQ Chicago (now WSCR on AM, but remaining in the call sign WMAQ-TV on television). The K sequence was also sequential, but with a more complicated history concerning the second letter; the third and fourth letters, however, were incremented in the most obvious way, with the fourth letter least significant, then the third letter, then the second letter. For both K and W, any call signs which were already assigned were simply skipped over in the sequential system.
The FCC has since begun requiring permit holders to choose the call sign prior to licensing; stations not yet given a call sign show up in FCC electronic records with the word "NEW" or the file number of the original station application instead. Stations may change their call signs whenever they wish to, and often they do so in connection with a change of radio format. Call signs become available again after 30 days of non-use, although stations (frequently under common ownership) can swap call signs at the same time.
Records for officially deleted stations still remain in the electronic FCC database for some time, but with the last call sign prefixed with a "D", which is not allocated for valid station call signs in the United States; for example, a station whose last call sign was KXXX before deletion will appear in the database as DKXXX so as not to conflict with any station that may be assigned KXXX in the future.
Read more about this topic: Call Signs In North America
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