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.
A distress signal is an internationally recognized means for obtaining help. Distress signals take the form of or are commonly made by using radio signals, displaying a visually detected item or illumination, or making an audible sound, from a distance. A distress signal indicates that a person or group of people, ship, aircraft, or other vehicle is threatened by grave and imminent danger and requests immediate assistance. Use of distress signals in other circumstances may be against local or international law.
In order for distress signalling to be the most effective, two parameters must be communicated:
For example, a single aerial flare alerts observers to the existence of a vessel in distress somewhere in the general direction of the flare sighting on the horizon but extinguishes within one minute or less. A hand held flare burns for 3 minutes and can be used to localize or pinpoint more precisely the exact location or position of the party in trouble. An EPIRB both notifies or alerts authorities and at the same time provides position indication information.