The SOS signal celebrates 100 years of official use today.
It took the tragedy of the Titanic to reveal just how vital a universal system was. After the collision in April 1912, the ship's radio operators sent out both the old CQD and the new SOS signals, but some ships in the area ignored both, thinking that they were having a party. They soon learnt otherwise, as international headlines told how Jack Phillips, the Titanic's first radio operator, and 1,500 others had been lost along with the "unsinkable" ship. The new SOS distress signal was rarely ignored after that.
Guglielmo Marconi gave testimony to the panel investigating the loss of the Titanic about the emergency signals.
Mr. Marconi explained the distress signals in use in vessels equipped with wireless telegraphy. "C.Q." meant "All stations" and "C.Q.D." was the distress signal. According to the regulations that signal must not be used except by order of the captain of the ship, or other vessels transmitting the signal. Since 1908 the distress signal had been "S.O.S." This and the "C.Q.D." were simply three letters, but they could be interpreted as meaning "Come quickly, danger," and "Save Our Souls".
Here's a simulation of the message that the Titanic sent out that night.