SOS in Morse code

How to read it

The service regulations of the International Wireless Telegraph Convention, signed in Berlin on 3 November 1906, say: “Ships in distress shall use the following signal”, print the dots and dashes run together, and add “repeated at brief intervals.” The regulation gives the signal no letters; it calls it “the signal of distress”. The ITU says that conference, the first International Radiotelegraph Conference, established SOS in Morse code as the international maritime distress signal. ARRL’s glossary defines SOS as a Morse code call for emergency assistance.

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One signal, not three letters

Split into three equal parts, the nine elements spell S, O and S in International Morse, which is where the name comes from, as Thomas H. White’s history of early American radio explains it. Britannica describes the signal the same way, as three dots for S and three dashes for O. Sent as the regulation prints it, the signal has no gaps between the letters, where S, O and S sent as three letters would be three units apart.

What the 1906 rules required

Article 9 of the convention binds wireless stations “to give absolute priority to calls of distress from ships”. Under Article XVI of its service regulations, a station that hears the signal stops all other correspondence until the distress traffic is over. If the ship adds a particular station’s call letters, only that station answers; otherwise every station that receives the call must. The convention went into effect on 1 July 1908. White notes that Germany had adopted the same run-together signal for distress in 1905.

The ITU says the sinking of the Titanic in 1912 showed the need for further improvements: months later its 1912 conference in London agreed on a common wavelength for ships’ distress signals.

CQD and “S 5 S”

Marconi operators resisted the international signal for a number of years and kept their company’s own distress call, C.Q.D., according to White. In 1912 American Morse was still common in the United States, including on the Great Lakes, and there three dashes are the figure 5, so writers of the time called the signal “S.5.S.” White points out that the signal itself was the same continuous string either way. See American Morse code and, for the general call CQ, CQ in Morse code.

SOS and MAYDAY

ARRL’s glossary gives SOS as the call for emergency assistance in Morse code and MAYDAY, from the French m’aidez (help me), as the call in voice modes.