An SOS is a distress call which has been internationally recognized since 1906. The use of SOS as a distress signal emerged in the early days of radio communications, when ships wanted to be able to quickly and accurately signal each other to ask for assistance. In addition to being signaled via radio, an SOS can also be signaled using lights, which can be manipulated to spell out words and code terms in International Morse Code; on a radio which allows voice communications, people may use SOS or other terms like “mayday” to indicate that they are in trouble on ships, aircraft, and other vessels.
In Morse Code, SOS is written out as “...---...” and it sounds like “dit dit dit dah dah dah dit dit dit.” Typically, an SOS is signaled with no spaces between the letters, creating a continuous signal, and this differentiates it from other Morse transmissions. It is also very easy to identify, even if a transmission is picked up in the middle, because by convention people repeat an SOS several times, with small breaks between the individual transmissions. Even in situations where voice communications are available, a transmission may include SOS calls so that people just tuning it are aware that an emergency is unfolding.
The German government was the first to adopt SOS, in 1905, and it was picked up by the international community a year later. There are several advantages to using SOS as a distress signal. For one thing, it is very easy to remember, transmit, and comprehend. Also, by choosing a unique signal, the international community ensured that an SOS would stand out from other transmissions, and because the SOS is internationally recognized, ships of any nationality can come to the assistance of a troubled vessel.
Contrary to popular belief, “SOS” does not stand for anything, although a number of backronyms like “save our souls” and “save our ship” have been formed from “SOS.” This code was chosen solely on the basis of the fact that it was easy to transmit. Incidentally, an SOS is also visually readable in any direction, which can be convenient when it is written out.
Prior to the introduction of SOS, many ships signaled distress with the code “CQD,” followed by the call sign of the ship. “CQ” stands for “attention all,” while the “D,” as you might imagine, means “distress.” People also sometimes used the code “NC,” which is derived from the International Code of Signals.