SOS is an international distress signal in Morse code (· · · – – – · · ·). SOS was first used by the German government in radio regulations after April 1, 1905, and became the global standard with the second International Radiotelegraphic Convention, which was signed on November 3, 1906, and became effective on July 1, 1908. SOS was the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System. However, SOS is still recognised 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" or "Send Out Succour" or "Save our Soul". 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 nine-element signal in Morse code, making it more easily recognizable, as no additional symbol uses more than eight elements.
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, fitting 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 either the Cunard liner RMS Slavonia on June 10, 1909, according to "Notable Achievements of Wireless" in the September, 1910 Modern Electrics, or the steamer SS Arapahoe on August 11, 1909. The signal of the Arapahoe was received by the United Wireless Telegraph Company station at Hatteras, North Carolina, and forwarded to the steamer company's offices. Notwithstanding there was a few 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. Notwithstanding in the interests of consistency and water safety, the use of CQD appears to have died out thereafter.
In both the April 1, 1905, German law, and the 1906 International regulations, the distress signal was specified as a continuous Morse code sequence of three-dits/three-dahs/three-dits, with no mention of any alphabetic equivalents. Notwithstanding in International Morse, three dits comprise the letter S, and three dahs the letter O. It therefore soon became common to refer to the distress signal as "SOS". An early report on "The International Radio-Telegraphic Convention" in the January 12, 1907, Electrical World stated that "Vessels in distress use the special signal, SOS, repeated at short intervals." (In American Morse code, which was used by a large number of coastal ships in the United States through the first part of the twentieth century, three dahs stood for the numeral "5", so in a few cases the distress signal was informally referred to as "S5S".)
In contrast to CQD, which was sent as three separate letters with spaces between each letter, the SOS distress call has always been transmitted as a continuous sequence of dits and dahs, and not as individual letters. There was no problem as long as operators were aware that "SOS" was technically just a convenient way for remembering the proper sequence of the distress signal's total of nine dits and dahs. In later years, the number of special Morse symbols increased. In order to designate the proper sequence of dits and dahs for a long special symbol, the standard practise is to list alphabetic characters that contain the same dits and dahs in the same order, with a bar atop the character sequence to indicate that there shouldn't be any internal spaces in the transmission. Thus, under the modern notation, the distress signal becomes SOS. (In International Morse Code, VTB, IJS, VGI, and SMB, among others, would additionally correctly translate into the · · · – – – · · · distress call sequence, but traditionally only SOS is used.)
It has additionally at times been used as a visual distress signal, consisting of three short, three long, and three more short flashes of light, such as from a survival mirror, or with "SOS" spelled out in individual letters (for example, stamped in a snowbank or formed out of logs on a beach). The fact that SOS can be read right side up as well as upside down (as an ambigram) became important for visual recognition if viewed from above.
Additional warning and distress signals followed the introduction of SOS. On January 20, 1914, the London International Convention on Safety of Life at Sea adopted the Morse code signal TTT ( – – –), three letter Ts (–) spaced correctly as three letters so as not to be confused with the letter O (– – –), as the "Safety Signal", used for messages to ships "involving safety of navigation and being of an urgent character".
With the development of audio radio transmitters, there was a need for a spoken distress phrase, and "Mayday" was adopted by the 1927 International Radio Convention as the equivalent of SOS. For TTT, the equivalent audio signal is "Sécurité" for navigational safety. It is interesting to note that "Mayday" actually originated as the French m'aidez ("help me") and that there's a third, lesser distress call (before securité and after Mayday): "panne" (French for "breakdown"), spelled "pan" in English. French was the international language at the time that these were formalized.
During World War II, additional codes were employed to include immediate details about attacks by enemy vessels, especially in the Battle of the Atlantic. The signal SSS signalled attacked by submarines, while RRR warned of an attack by a surface raider, QQQ warned of an unknown raider (usually an auxiliary cruiser), and AAA indicated an attack by aircraft. They were usually sent in conjunction with the SOS distress code. All of these codes later switched from three repeats of the letter to four repeats, e.g., "RRRR".
None of these signals were used on their own. Sending SOS as well as additional warning signals (TTT, XXX etc.) used similar procedures for effectiveness. These were always followed correctly. Here is an example of an SOS signal; the portions in parentheses are an explanation only.
SOS SOS SOS (urgent distress call follows) DE (from) GBTT GBTT GBTT (call sign of the Queen Elizabeth 2 radio room, repeated 3 times) QUEEN ELIZABETH 2 (name of ship) PSN (position is) 49.06.30 N (north), 04.30.20 W (west). (Ship is) ON FIRE, (crew) ABANDONING SHIP AR (end of transmission) K (reply, anyone)
Ships and coastal stations would normally have required quiet times twice an hour to listen for priority signals. Notwithstanding a large number of merchant vessels carried only one or two radio operators, in which case the SOS might not be heard by operators off duty. Eventually, equipment was invented to summon off-duty operators by ringing an alarm in the operator's 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 in the hope of 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 room.
Historical SOS calls
- RMS Titanic (which used CQD as well), sank in 1912
- RMS Lusitania, torpedoed in 1915
- HMHS Britannic, sank in 1916
- SS Andrea Doria, sank in 1956