SOS in Morse Code — Timing, Sound, and History

The answer depends on whether you send SOS as three separate letters or as the continuous distress prosign. Most charts show the three-letter form, ... --- ..., with a three-unit gap between the letters. The actual distress signal is sent as one group, ...---..., with only the one-unit gaps inside the pattern.

The calculation uses the PARIS standard: one unit is 1.2 / WPM seconds. At 20 WPM one unit is 60 ms, so the three-letter form is 27 × 60 ms = 1.62 seconds. At 12 WPM the unit is 100 ms, and the same 27 units take 2.70 seconds.

The player above uses the three-letter form because it is the version most people expect to hear. If you are practising the real prosign, remove the spaces and keep the gaps inside the pattern at one unit.

SOS was not chosen as an abbreviation. The 1906 International Radiotelegraphic Convention in Berlin proposed a short, unmistakable distress signal, and SOS became the standard in 1908, replacing the earlier CQD call. The pattern is symmetric, easy to recognize in noise, and short enough to repeat without losing the rhythm.

“Save Our Souls” and “Save Our Ship” came later. They are memorable backronyms, not the origin of the signal. The letters were chosen for the sound of the pattern, not for the words they could spell.

The most famous use came in 1912, when the Titanic's operators sent CQD first and then SOS as the ship sank. The signal was heard by nearby ships, including the Carpathia, which rescued more than 700 people. SOS later became the international standard for maritime distress and remains a recognized signal today.

Any on/off signal can carry SOS: a radio transmitter, a signal lamp, a whistle, a car horn, a torch or a finger tapping on a table. The medium does not matter; the rhythm does.

• Radio: key three dots, three dashes, three dots. Keep the gaps inside the pattern at one unit and repeat after a short pause.
• Light: three short flashes, three long flashes, three short flashes. A long flash is three times the length of a short one.
• Sound: the same short-long-short rhythm on a whistle or horn. If you are using the three-letter form, leave a clear gap between the letters.
• Touch: tap the rhythm on a surface or use the phone's vibration motor if you are building an accessibility tool.

SOS is a distress signal. Do not send it on a monitored emergency frequency, to a rescue service, or in a situation where it could be mistaken for a real emergency unless you actually need help.

The most common mistake is closing the gaps. If you send the three-letter form without a three-unit gap between the letters, the receiver hears one long unknown code instead of SOS. The second mistake is the opposite: leaving so much space that each dot or dash sounds like a separate character.

Another common problem is mixing International Morse with American Morse. The two systems use different codes for some letters and different spacing inside some characters. SOS is an International Morse signal; a historical railroad message may use the older system and will not decode with a standard International table.

Finally, do not assume the audio is wrong if it sounds faster than you expected. At 20 WPM a dot is only 60 milliseconds long. Use the speed slider on the player to slow it down to 10 or 12 WPM while you learn the rhythm.

SOS is the Morse code distress signal. MAYDAY is the voice distress call, derived from the French “m'aidez” (help me), and it is used on radiotelephone channels. They are not interchangeable: SOS belongs to Morse and other on/off signalling; MAYDAY belongs to voice radio.

The Global Maritime Distress and Safety System replaced the mandatory Morse watch at sea in 1999, but SOS is still used in amateur radio, in emergency beacons and in everyday signalling. Its rhythm is one of the few pieces of Morse code that most people can recognize without training.

To hear the individual letters, open the Morse code alphabet. To build your own message, use the Morse code generator or decode a reply with theMorse code decoder.