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 (), as the "Safety Signal", used for messages to ships "involving safety of navigation and being of an urgent character".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". Sending SOS as well as other warning signals (TTT, XXX etc.) used similar procedures for effectiveness. Here is an example of an SOS signal; the portions in parentheses are an explanation only.In modern terminology, SOS is a Morse "procedural signal" or "prosign", 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.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.
These were sent prior to the SOS in the hope of ringing the automatic alarm in ships so equipped.The SOS distress signal is a continuous spaceless sequence of three dots, three dashes, and three dots.In International Morse Code, three dots form the letter S, and three dashes make the letter O, so "SOS" became an easy way to remember the order of the dots and dashes.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 operator's berth.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, becoming effective on July 1, 1908.