SOS is a distress call, it is recognized internationally as a message calling for help in a time of extreme distress. The letters SOS are not an acronym, SOS was developed to be used as a radio signal. In Morse code, “s” is transmitted as a series of three dots and “o” is transmitted as a series of three dashes, which makes SOS in Morse code easy to transmit repeatedly and easy for the listener to decode. Germany was the first country to used SOS as a distress call in 1905, the SOS distress call was adopted as the universal standard in 1906. Today, SOS is often spelled out as a signal of distress by people who are stranded in the wilderness as it is short and universally understood. SOS is sometimes seen rendered with periods as in S.O.S., but the letters do not represent any actual words and the form approved by the Oxford English Dictionary is SOS.
Mayday is a distress call, it is recognized internationally as a message calling for help in a time of extreme distress. Mayday is generally spoken over the radio by people on ships or on airplanes. The word Mayday was invented by Frederick Stanley Mockford, the senior radio officer at Croydon Airport in London, England in 1923. Mockford was tasked with coming up with a term to indicate distress that would be easily recognizable. Since much of the air traffic to Croydon Airport came from Le Bourget Airport in Paris, France, Mockford settled on Mayday, an Anglicization of the French m’aider which means help me. When Mayday is used in an emergency situation it is repeated three times as in Mayday, Mayday, Mayday. The Oxford English Dictionary capitalizes the term, but it is sometimes seen with a lower case letter as in mayday.
Examples
Nursery hit by six feet of water sends SOS on Facebook (The Birmingham Mail)
Agra: The Wildlife SOS Rapid Response Unit rescued an injured peacock from a near death situation from Khandoli village, Agra. (The Times of India)
As a steady stream of 120 golfers arrived for their tee-off times at Invergordon Golf Club, the seven members of the duty crew left to launch the lifeboat in response to a Mayday call from a small fishing vessel in the Cromarty Firth. (The Ross-Shire Journal)
The incident unfolded around 5:30 p.m. when the U.S. Coast Guard received a mayday call after the 40-foot boat struck an unidentified object near Bloodsworth Island, an area north of the town of Crisfield in Dorchester County. (Stars and Stripes)