A doctrine was developed of having units in the field, particularly ships at sea, maintain radio silence except for urgent situations, such as reporting contact with enemy forces.
During World War II, the BBC would include "personal messages" in its broadcasts of news and entertainment to occupied-Europe.
[citation needed] In the mid-20th century, the High Frequency radio bands were used by numerous stations sending seemingly random Morse code, usually in five-letter groups.
Though there has been no official confirmation (beyond a 1998 article in The Daily Telegraph which quoted a spokesperson for the Department of Trade and Industry as saying, "These [numbers stations] are what you suppose they are.
"[1]) there is little doubt that most of these numbers stations are primarily used to send messages to spies and other clandestine agents (additional possible uses include communication with embassies when a crisis might dictate destruction of cryptographic equipment and as a backup to normal command systems in wartime).