Why do USA radio stations have 'W' or 'K' in the name?

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Mystery Hour Question

Why do so many American radio stations have ‘W’ or ‘K’ in their name?
Chris, Chelmsford

Answer

Name: Brian, Hornchurch
Qualification: Read it in a book somewhere once

Answer: Because of regulations issued by the FCC in the United States in order to differentiate between different stations. Stations with ‘W’s in the name are East of the Mississippi, whereas stations with ‘K’ are West of the Mississippi.