According to the Wikipedia entry on this term, “Mayday is an emergency code word used internationally as a distress signal in voice procedure radio communications. It derives from the French venez m'aider, meaning 'come help me'. It is used to signal a life-threatening emergency by many groups, such as police forces, pilots, firefighters, and transportation organizations. The call is always given three times in a row ("Mayday Mayday Mayday") to prevent mistaking it for some similar-sounding phrase under noisy conditions, and to distinguish an actual Mayday call from a message about a Mayday call.”
But mayday is not the only alert signal of that kind. In fact, that is the one which indicates closer danger and biggest urgency. Pan-pan (from the French: panne - a breakdown) is used for urgent situations of a lower order , such as a mechanical breakdown. Finally, Securite (from French sécurité — safety) introduces an important safety information, such as navigational warnings or the approaching of meteorological adverse conditions.
These three signals have Morse equivalents in SOS (• • • — — — • • •), XXX (— —••— — — —••— — — —••— — ) TTT ( — — —)
Regarding SOS, I read that its association with phrases such as "Save Our Souls" were developed after the signal, most likely as a means to help remember the correct letters (something known as a backronym).
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Never heard about backronyms, but they are fun! I like this one: TWAIN (Technology Without Any Interesting Name).
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