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SOS is the commonly used description for the international Morse code distress signal (· · · — — — · · ·). This distress signal was first adopted by the German government in radio regulations effective April 1, 1905, and became the worldwide standard under the second International Radiotelegraphic Convention, which was signed on November 3, 1906 and became effective on July 1, 1908. SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress Safety System.[1] SOS is still recognized as a visual distress signal.[2]

From the beginning, the SOS distress signal has really consisted of a continuous sequence of three-dits/three-dahs/three-dits, all run together without letter spacing. In International Morse Code, three dits form the letter S, and three dahs make the letter O, so "SOS" became an easy way to remember the correct order of the dits and dahs. In modern terminology, SOS is a Morse "procedural signal" or "prosign", and the formal way to write it is with a bar above the letters, i.e. SOS.

In popular usage, SOS became associated with phrases such as "save our ship" or "save our souls". These were a later development, most likely used to help remember the correct letters (a backronym). As the SOS signal is a prosign, its respective letters have no inherent meaning per se, it was simply chosen due to it being easy to remember.[citation needed]



The use of the SOS signal was first introduced in Germany as part of a set of national radio regulations, effective April 1, 1905. These regulations introduced three new Morse code sequences, including the SOS distress signal.

In 1906, at the second International Radiotelegraphic Convention in Berlin, an extensive collection of Service Regulations was developed to supplement the main agreement, which was signed on November 3, 1906, becoming effective on July 1, 1908. Article XVI of the regulations adopted Germany's Notzeichen distress signal as the international standard, reading: "Ships in distress shall use the following signal: · · · — — — · · ·  repeated at brief intervals". The first ship to transmit an SOS distress call appears to have been the Cunard liner Slavonia on June 10, 1909, according to "Notable Achievements of Wireless" in the September, 1910 Modern Electrics. However, there was some resistance among the Marconi operators to the adoption of the new signal, and, as late as the April, 1912 sinking of the RMS Titanic, the ship's Marconi operators intermixed CQD and SOS distress calls. However, in the interests of consistency and water safety, the use of CQD appears to have died out after this point.

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