On 3 October 1906 the 'SOS' signal was established as an International Distress Signal by an agreement made between the British Marconi Society and the German Telefunk organisation at the Berlin Radio Conference. The signal was formally introduced on 1 July 1908.
In Morse code SOS signified by three dots, three dashes, then three dots (· · · – – – · · ·). SOS was chosen because it could not be misinterpreted as being a message for anything else.
The first time the 'SOS' signal was used in an emergency was on 10 June 1909, when the Cunard liner SS Slavonia was wrecked off the Azores. Two steamers received her signals and went to the rescue. Tree years later the Titanic used it to signal the sink of the ship.
The CQD signal was devised by the Marconi Company and was intended to mean 'All Stations - Urgent', but was popularly misinterpreted as 'Come Quick - Danger' or 'Come Quickly Down'.
The SOS was actually brought into use in 1908, but it's adoption by maritime stations was very slow. So slow in fact that at the time of the sinking of the Titanic on the 15th of April 1912, her radio operator, Harold Bride, used both the old 'CQ Distress' and the then newer SOS signals.
As an aside, the SOS combination of letters was chosen because the "di di dit, dah dah dah, di di dit" sound was easily recognizable. It was only later that the suggestion was made that it might stand for 'Save Our Souls'.
Also on this day......
3 October 1990 : East and West Germany re-united and became one country.