SOS does not stand for anything
Save Our Souls and Save Our Ship are both backronyms, invented after the fact because the letters looked like they ought to mean something.
The signal was chosen at the 1906 Berlin radiotelegraph conference and came into force in 1908 for one reason: three dots, three dashes, three dots is a rhythm nobody mistakes for anything else, even through heavy interference. It was picked for its sound, not its spelling.
It is one signal, not three letters
Written properly, SOS carries a bar over the top to show it is sent without the usual gaps between letters. Leave normal letter gaps and you have simply sent the letters S, O and S.
Sent correctly it is nine signals in an unbroken run, which is exactly what makes it stand out against noise.
Signalling SOS with light or sound
The pattern is the same whatever you use to send it. Three short, three long, three short, then a pause before repeating.
- With a torch: three quick flashes, three slow flashes, three quick flashes.
- With a whistle: three short blasts, three long, three short. Sound carries further than a shout and costs far less breath.
- With a mirror: aim reflected sunlight at your target and interrupt it in the same rhythm.
- By tapping: on a pipe or a wall, which is how trapped people are most often located.
Repeat with a clear pause between each round. The pause is what tells a rescuer they are hearing a deliberate pattern rather than random noise.
Related signals worth knowing
CQD was the earlier distress call and was still in use alongside SOS on the Titanic in 1912. Mayday replaced it for voice radio, chosen because it survives a bad connection. PAN-PAN is the level below: urgent, but not immediately life threatening.
When not to send it
SOS means life is at risk right now. Sending it as a joke or a test wastes the time of people who launch boats and helicopters on the strength of it, and in most countries it is a criminal offence.