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A Morse code call for emergency assistance.
The American delegation suggested the letters NC which were already recognized in the International Signal Code for Visual Signalling. The German delegation proposed its own SOE, which was already in use on German ships as a general inquiry signal similar to CQ (which was then used only by the Marconi system). The British delegation wanted the Marconi signal CQD.
The convention found SOE acceptable except that the final E could easily be lost in QRN so the letter S was substituted, making it SOS. The convention decided that SOS should be sent as a single code character with a sound unlike any other character, thus arresting the attention of anyone hearing it. In 1912, after the Titanic disaster, SOS became universal.
source: ARRL Ham Radio Glossary
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