From the FCC website’s page on Station Identification and Call Signs:
[quote]
HOW CALL SIGNS ARE DESIGNATED
Since the early days of wireless telegraphy, starting with marine use, radio stations have had their own identification. Under international agreement, since 1927 the alphabet has been divided among nations for basic call sign use. The United States, for example, is assigned three letters–N,K, and W-- to serve as initial call letters for the exclusive use of its radio stations. It also shares the initial letter A with some other countries. The letter A is assigned to the Army and Air Force; N to the Navy and Coast Guard, and K and W to domestic stations, both government and non-government.
The Communications Act gives the FCC authority to designate call letters to all United States radio stations. This is done on an individual station basis. A and N block assignments are designated for government use. Further details on the FCC’s requirements for the identification of radio stations may be found in the Code of Federal Regulations, Title 47, Part 2, (Subpart D) of the rules. Some types of radio stations and equipment, such as radar stations and diathermy equipment, are exempt from such requirements.
BROADCAST STATION IDENTIFICATION
Broadcast stations in this country are assigned call signs beginning with K or W. Generally speaking, those beginning with K are assigned to stations West of the Mississippi River and in U.S. territories and possessions, while those beginning with W are assigned to broadcast stations East of the Mississippi River. During radio’s infancy, most of the broadcast stations were in the East. As stations began operating, the Mississippi became the dividing line between K and W call signs. The few exceptions to existing call signs within this system were assigned before the allocation plan was adopted. Station KDKA, Pittsburgh, Pennsylvania, is one example.
Since the beginning of broadcasting, stations have had the privilege of requesting specific call signs. In requesting their preferences for certain letters of the alphabet, broadcasters have presented combinations of names, places or slogans. For example, the letters NBC are used for stations owned by the National Broadcasting Company, CBS for those of the Columbia Broadcasting System, and ABC for the American Broadcasting Companies. Examples of individual station call letters are: WGN, Chicago (“World’s Greatest Newspaper”), WNYC, New York (New York City municipal station); KAGH, Crossett, AK, (“Keep Arkansas Green Home”); WIOD, Miami (“Wonderful Isle of Dreams”); WLS, Chicago (“Worlds Largest Store”); WACO, Waco Texas); WTOP, Washington, D.C. (“Top of the Dial”); KFDR, Grand Coulee, Washington, (Franklin D. Roosevelt); WCFL, Chicago (“Chicago Federation of Labor”); WMTC Vancleve, KY, (“Win Men to Christ”); WGCD, Chester, S.C. (“Wonderful Guernsey Center of Dixie”); Educational TV station WXXW, Chicago, uses the Roman numerals for its channel 20; and KABL, Oakland, CA, selected its letters to represent San Francisco’s famous cable cars. If a new broadcast station makes no specific request, it is assigned a call sign by the FCC. Since 1946 the FCC has not guaranteed specific call signs to be granted prior to the grant of a construction permit or special temporary authority.
As broadcast stations began to increase in the early 1920’s, the three letter call sign could no longer accommodate the growing number of stations, making it necessary to add a fourth letter.
With the advent of FM and TV in 1941, new call signs for all such stations were not assigned. Rather, since many FM and TV stations were operated by the same AM licensee at the same license area, the general practice was for the associated FM or TV station to simply add “-FM” or “-TV”, to the call sign of the co-owned AM station. International Radio Regulations do not require the use of call signs by broadcast stations if some other suitable means of identification is employed. For example, many foreign stations identify by announcing, "The Voice of … "or “Radio…”