SOS does not stand for save our souls or save our ship. It was chosen for its rhythm: three dots, three dashes, three dots sent as one unbroken signal. Before wireless telegraphy in the late 1890s, ships at sea had only flags, flares, and foghorns. Early spark gap radio brought a new problem, corporate rivalry: the Marconi Company forbade its operators to answer ships using competing equipment. In 1903 Italian delegate Quintino Bonomo proposed SSSDDD, in January 1904 Marconi mandated CQD for its stations, and the U.S. Navy pushed NC from the flag code. Germany introduced the continuous three dot, three dash, three dot signal in 1905, and the 1906 International Radiotelegraph Convention in Berlin adopted it. Operators called it SOS as a memory aid.

Habits lingered. Americans using American Morse, where three dashes meant 5, called it S5S, and Marconi men clung to CQD until the Titanic, where Jack Phillips and Harold Bride alternated between CQD and SOS. The 1927 convention added spoken Mayday, from French for help me, along with pan-pan and securite. In World War II operators appended codes naming the attacker, and auto alarms let off duty crews be woken. The Morse SOS was officially replaced by the satellite based GMDSS in 1999, but it survives as an ambigram that reads the same from any angle.

• The overscored SOS is a prosign, sent with no pauses between letters.
• Wartime suffixes were SSS for submarine, RRR for surface raider, AAA for aircraft, and QQQ for unknown.
• Triggering an auto alarm took 12 dashes of exactly four seconds with one second gaps.
• Radio room clocks carried painted red bands and wedges marking the dashes and the three minute silent periods held twice an hour.
• The radiotelephone alarm alternated between 2,200 and 1,300 hertz.