Numbers stations are coded messages broadcast at specific frequencies over shortwave radio. They sometimes begin and end with a few bars of music, chimes, beeps or bells that can become signatures used for identifying different broadcasters.
The content of numbers station messages is usually a single voice, either human or computerized, reciting a series of numbers or letters. The sender is never identified, nor are the location, intended receiver or code key.
Numbers stations became more prominent in the 1970s and throughout the Cold War. Avid numbers station listeners believe that they are a means of communication between intelligence agencies and spies in the field.