<P> Amateur radio stations in Mexico use "XE1" for the central region, "XE2" for the northern region, and "XE3" for the southern region . "XF" prefixes indicate islands . "XF4" is usually used for the Revillagigedo Islands and nearby islets . Special call signs for contests or celebrations are occasionally issued, often in the 4A and 6D series, although these will follow the usual district numbering system (4A3 for the south, etc .). </P> <P> 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 Pittsburgh, Pennsylvania, which became KDKA in November 1920, and Charles Herrold's series of identifiers from 1909 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 onward . </P> <P> 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 . Initial letters "AA" through "AL", as well as "N", are internationally allocated to the United States but are not used for broadcast stations . </P> <P> 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 . </P>

What does the w stand for on tv stations
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