SOS
Additional warning and distress signals were introduced after SOS. On January 20, 1914, the London International Convention on Safety of Life at Sea adopted the Morse code signal TTT ( —  —  —), three letter T's (—) 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." When radio transmitters that could send audio were developed, 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 signals are "Pan-pan" for urgency and "Securite" for navigational safety.