For ships at sea and planes, for a long time, the preferred means of communication was Morse code. Morse Code uses dots and dashes to communicate messages using the English alphabet. In the late 1800s, each country had its own distress signal, and it was at a maritime convention in 1905 that SOS was agreed upon.
Unlike the urban legend, SOS does not stand for save our souls. It was picked because on morse code SOS is … - - - … which is impossible to miss, easy to type and simple to commit to memory. Also, it is language agnostic.
This remained the distress signal for 2 decades, and then radio showed up.
How many times have you tried to spell something out over the phone, and the listener confused the P with T or S with H? Listening to specific words over radio static was very difficult. In the early days of radio, it was not that clear either.
In 1923, Fredrick Stanley Mockford, working at Croydon Airport as the senior radio officer, coined Mayday. At the time, the traffic between France and England was rising. In French, “help me” is m’aider. He took the phonetic equivalent in English. Since almost all of the air traffic between the two countries at the time was between Croydon Airport and Le Bourget, his proposal had a huge impact.
It became standard protocol to say mayday, mayday, mayday, three times in the event of an emergency, just in case the radio garbled part of the signal.
In 1927, the International Radiotelegraph Convention in Washington DC adopted the voice call “Mayday” as the radiotelephone distress call.