Although some people will tell you that this is just a convenient acronym made up to fit with the Morse code letters (which are � � � / - - - / � � � ). It's often said that this was the easiest message to send by Morse Code because there is little discrepancy as to what it could mean (you can't mistake three dots, followed by three dashes and then another three dots as something else - similarly, you can't type/send it by accident). This combination of dots and dashes equated to an S, an O and then another S so people made up a suitable acronym to fit.