Following a practice inaugurated in 1912 when the federal government first licensed radio stations, beginning in 1921 broadcasting stations have generally been assigned call signs beginning with "K" when their community of license is located in the west, and with "W" in the east. (The FCC proposed ending this policy in 1987, to allow unrestricted assignments of K and W stations throughout the country. However, it was decided to retain the rule due to industry objections.)

Source: Wikipedia