SOS

Although thought by some to mean Save Our Souls, in actuality the signal was agreed upon because it was easy enough for even an amateur to use or recognize, even with interference.

The original radio distress signal was CQD. proposed by Marconi and adopted in 1904 the CQ was a general attention notice with the D for distress. It was popularly rendered as "come quick, distress". The CQD signal did not last long, at an international conference in Berlin in 1906 the German standard SOS was adopted and officially ratified in 1908. The CQD signal remained in use for a few more years, especially with British operators who had first proposed it.

The first significant rescued following a radio distress signal was in January 1909 when 1500 people were recovered from the collision of the Republic and the Florida by the Baltic after it had heard the CQD message.

Its most famous first major use was with the sinking of The Titanic when the radio operators used it in conjunction with the older distress signal.

Urgent but not emergency calls used the prefix XXX.