For alternative uses, see SOS (disambiguation).

SOS is the international Morse code distress signal. It consists of three dots, three dashes and three dots (· · · - - - · · ·). This is the same as for the letters S, O and S, except that it is sent as one single letter - that is, with the inter-letter spaces converted to inter-element spaces.

Although thought by some to mean Save Our Souls or Save Our Ship, in actuality the signal was agreed upon because it was easy enough for even an amateur to use or recognize, even with interference. In addition, the fact that the signal is sent as one letter rather than three separate signals indicates that it is not an abbreviation.

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." Urgent but not emergency calls used the prefix XXX. Announcements relating to safety at sea used the prefix TTT. 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 rescue 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. The most famous early major use of SOS was with the sinking of RMS Titanic on April 15, 1912, when the radio operators on board the ship used it in conjunction with the older distress signal.

See also

• Mayday
• Distress signal
• Call for help
• 500 kHz