For most of us, SOS is probably as far as it goes for our Morse code knowledge. Those three letters have become a word in their own right and we could all probably tap them out if things got sufficiently sticky but what do they actually stand for in the first place?
The most widely-held misconceptions are ‘save our souls’, ‘save our ship’ and other variants. Another clever little suggestion is ‘si opus sit’ which comes from Latin and is used in medicine to mean ‘give relief where necessary’.
The truth is that SOS doesn’t actually stand for anything. The letters were simply chosen because they were easy to send in an emergency and difficult to mistake: three dots, three dashes, three dots.
From 1836 onwards, Samuel F. B. Morse and his colleagues Joseph Henry and Alfred Vail developed an electrical telegraph system which could send pulses of electrical current along a wire. Language could then be transmitted using the pulses and the silence between them. Morse code was developed over the next few years and, interestingly, Vail assigned each letter its dots and dashes based on usage. He determined the frequency of each letter in English by counting the movable type found in type-cases of a local newspaper. Therefore, e, being the most common letter, is simply one dot.
Originally, telegraphy was designed to make indentations onto a paper which operators could then read, however, it was later found that people were more proficient at receiving Morse code when it was taught as a language that is heard rather than read.
From the 1890s onwards, it was used extensively for radio communication, before it was possible to transmit the sound of a voice and it was vital in World War II for carrying messages between war ships and naval bases.
An alternative to SOS which was suggested when looking for a Morse code distress signal was CQD or CQ meaning ‘come quickly, distress’, which would have given a far less elegant – · – · – – · – – · · But both were used for a time. In fact, during the sinking of HMS Titanic in 1912, operators used both SOS and CQD as distress calls.
SOS was first introduced in 1908 and remained the maritime distress signal until 1999.