SOS in Morse code
... --- ...
The international distress signal: three dots, three dashes, three dots. It was adopted at the International Radiotelegraph Convention in Berlin in 1906 and came into force in 1908. It was chosen because it is easy to send and hard to mistake, not because the letters stand for anything; "save our souls" came later. Operators send it as one unbroken signal, ...---..., without the gaps between the letters. It works as light and sound too: three short, three long, three short.