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. SOS is still recognized as a visual distress signal.
The SOS distress signal is a continuous sequence of three dits, three dahs, and 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 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: SOS.
In popular usage, SOS became associated with such phrases as «save our ship», «save our souls» and «send out succour». These may be regarded as mnemonics, but SOS does not actually stand for anything and is not an abbreviation, acronym or initialism. In fact, SOS is only one of several ways that the combination could have been written; VTB, for example, would produce exactly the same sound, but SOS was chosen to describe this combination. SOS is the only 9-element signal in Morse code, making it more easily recognizable, as no other symbol uses more than 8 elements. Ships and coastal stations would normally have required quiet times twice an hour to listen for priority signals. However, many merchant vessels carried only one or two radio operators in which case the SOS may not be heard by operators off duty. Eventually equipment was invented to summon off-duty operators by ringing an alarm in the operators berth. This was triggered by the operator of the ship in distress transmitting twelve long dashes of four seconds duration each. These were sent prior to the SOS hopefully ringing the automatic alarm in ships so equipped. If possible a short delay was given before transmission of the SOS proper. This was to give those off watch operators time to get to their radio office.