Joel Achenbach's answer to the question of why all radio and TV station call letters begin with W or K {Style Plus, Jan. 4} scratches the surface of a great reserve of broadcasting lore.

In times of stricter regulation, the FCC forbade the use of any call sign that consisted of the initials of a U.S. president or a government agency, without ''suitable clearance.'' Sen. John Stennis (D-Miss.) assisted in clearing WTVA for a television station in Tupelo, and today an FM station in Chatham, Mass., uses WFCC. Washington listens to WJFK, but no one seems to have had much interest in, say, Herbert Hoover's initials.

The Mississippi River demarcation between W and K call signs is really more gray territory than a solid line. W's and K's jump the river banks from the Twin Cities on south. Some years back, the owner of KWK in St. Louis bought WGNU-FM, across the river in Granite City, Ill., expecting to call it ''KWK-FM.'' When the FCC refused permission, the station took WWWK instead. The AM-FM pair were promoted as KWK and ''3WK.'' The owner made a federal case out of it, and ultimately the courts -- noting that call signs were one of the least-known but highly regulated areas of FCC law -- ordered the FCC to permit KWK-FM to be used.

Call signs go through vogues much like high fashion. When ''easy listening'' formats came to FM in the early '70s, almost all of the EZ possibilities were snatched up. In the last decade, C, Q, R, X and Z have assumed popularity among rock stations in various combinations, as have L and T among ''light (or lite) rock'' stations.

Broadcasters place great value on call signs because they are a tangible expression of a station's intangible goodwill. Many are legendary acronyms, such as the Tribune Co.'s WGN, Chicago (''World's Greatest Newspaper''). WOW, Omaha, was operated by the Woodmen of the World Life Insurance Society (WOW's general manager in later years was a man named, ironically, William O. Wiseman). And, of course, WGMS is ''Washington's Good Music Station.''

Broadcasting has always been a highly competitive industry. Although probably apocryphal, the story goes that after seed company magnate Henry Field built a radio station in the Midwest earlier in the century, his chief -- and intense -- competitor, the May Seed Co., put a bigger and better station on the air and called it KMA. (Guess what that stands for.) It's a pretty popular station yet today in its home town of Shenandoah, Iowa.

JOHN WELLS KING Washington