SOS in Morse code: how to send it correctly
SOS in Morse code is ...---...: three dits, three dahs and three dits, sent as one continuous signal. Said aloud, it is di-di-di-dah-dah-dah-di-di-dit. It is a prosign, a single character of nine elements, so the only silences inside it are the one-unit gaps that sit between any two dits or dahs. There are no gaps between an S, an O and an S, because there are no separate letters.
That last point is the one most people miss. Below is the pattern element by element, how long it takes at different speeds, how to send it with sound or light, and how to practice it without anyone mistaking your practice for the real thing.
One thing first: if you are ever in trouble and can phone or text your local emergency number, do that. Knowing SOS is worthwhile; it is not a substitute for a call.
The pattern, element by element
Morse timing runs on one unit, the length of a dit. A dah is 3 units, and the silence between elements of one character is 1 unit. SOS uses only those three lengths:
| Part | Sound | Length |
|---|---|---|
| Elements 1 to 3 | dit, dit, dit | 1 unit each |
| Elements 4 to 6 | dah, dah, dah | 3 units each |
| Elements 7 to 9 | dit, dit, dit | 1 unit each |
| The 8 gaps between them | silence | 1 unit each |
| Total | 23 units |
The arithmetic: six dits make 6 units, three dahs make 9, and eight gaps make 8, for 23 in all.
Now compare the same dots and dashes sent as three letters, ... --- .... S is 5 units, O is 11, and each of the two letter gaps adds 3, which comes to 27. The difference is only four units, but it is the difference between one signal and three letters. The one-unit gaps hold a character together; the three-unit gaps split it into letters.
Why it is not S, O and S with gaps
A prosign is two or more letters run together into one character, with no letter gap. People name it by the letters you get if you split it, and write it with a bar over the letters or in angle brackets, like <SOS>. The letters are a label, not the signal.
You can see this by splitting the same nine elements in other places. Every one of these is exactly ...---... when the gaps are taken out:
| Split as | Code with letter gaps |
|---|---|
| S O S | ... --- ... |
| I J S | .. .--- ... |
| V T B | ...- - -... |
| S M B | ... -- -... |
| V 7 | ...- --... |
Nobody calls it IJS or V7, but the run of elements is identical. What makes it SOS is the unbroken rhythm, not the letters inside it. That is also why “Save Our Souls” is a later reading and not the origin; Why SOS? The real story behind the distress signal tells how the pattern was chosen and when it came into force.
In practice, someone who hears S, O and S with gaps will still understand what you meant. Sent as one run, though, it is the signal as defined, and it stands apart from ordinary text, where S and O turn up all the time. SOS is one of several prosigns; Morse code prosigns explained covers the others, from AR to SK.
How long it takes at different speeds
A dit lasts 1200 divided by the speed in words per minute, in milliseconds. Multiply by 23 and you have one SOS:
| Speed | Dit | Dah | One SOS (23 units) |
|---|---|---|---|
| 5 WPM | 240 ms | 720 ms | about 5.5 seconds |
| 12 WPM | 100 ms | 300 ms | 2.3 seconds |
| 20 WPM | 60 ms | 180 ms | about 1.4 seconds |
| By hand with a light, dit of 0.5 s | 500 ms | 1.5 s | about 11.5 seconds |
When you repeat it, leave at least a word gap between repeats: 7 units, which is longer than anything inside the signal. That silence tells the listener where one SOS ends and the next begins.
Sending it by sound
Anything that can hold a steady tone works: a whistle, a horn, a buzzer, a Morse key on a practice oscillator. The rules are the same whatever makes the sound:
- Hold the dahs. Each dah is three dits long. A common fault is three short dahs that sound almost like dits. Let them go long; a dah that is a little too long does far less harm than one that is too short.
- Keep the inside gaps short and even. One unit between every element, including at the two changes from dits to dahs and back. That is where nervous senders leave a pause and break the signal into letters.
- Say it in your head. “Di-di-di-dah-dah-dah-di-di-dit” at a steady pace gives your hand the rhythm. The syllables are the timing.
- Pause, then repeat. A clear silence of at least seven units, then the whole signal again.
Watch out for one failure in particular. If your dahs collapse into dits, ...---... becomes nine dits in a row, which sounds close to the Morse error signal of eight dits, the signal for “ignore that”. The Morse code error signal explains that one.
A knock or a tap cannot be held, so it cannot make a true dah. If all you can do is knock, the rhythm of the groups (three, then three, then three) is what carries; it is not standard Morse, and you should not count on it being read.
Sending it by light
If you need help, call your local emergency number first wherever you can. A light is something you might add to a call, never a replacement for one.
Light works the same way as sound: light on is a tone, light off is silence. A dit is a short flash, a dah is a flash three times as long, and the gaps follow the same 1, 3 and 7 pattern.
Light is usually sent much more slowly than sound. A hand on a switch is not quick, and watchers need time to tell a long flash from a short one. A dit of about half a second, as in the table above, is easy to keep steady.
A few practical points:
- Cover the lens rather than clicking the switch if your flashlight has a stiff or multi-mode switch. A piece of card over the light, or your hand if the light stays cool, gives cleaner on and off.
- Brace your arm so the beam does not wander between flashes.
- Never shine a light into anyone’s eyes, and never point one at a vehicle or an aircraft.
How to signal Morse code with a flashlight goes further into timing and visibility.
Practicing without raising an alarm
SOS is the first thing many people learn to send, and that is fine, but practice it where nobody could take it as real. Send it on a practice oscillator, a keyboard, a desk with your finger, or in your head. Never send it on a radio transmitter, and never flash it from a boat, a hillside or a window where someone might see it and act. Sending a false distress signal on the radio is against the law in many countries, and anywhere it can send rescuers out for nothing.
If you practice on a Mac, Dot & Dash starts here too: its first launch teaches E, one dit, and T, one dah, then has you send SOS as your first message, on a machine drawn on screen with a key, a tone and a paper tape that prints the dots and dashes you keyed. Seeing the tape is a quick way to tell whether your dahs were really dahs.
As for real trouble: SOS is still recognized almost everywhere as a sign of distress, but ships have not relied on Morse for distress since the Global Maritime Distress and Safety System (GMDSS) took over in 1999, and a phone call reaches help far faster than any signal a stranger has to notice and decode. If you can call, call.
Questions
Is SOS three letters?
No. It is a single prosign of nine elements, ...---..., sent with no letter gaps. The letters S, O and S are just a convenient name for the pattern.
Does SOS stand for Save Our Souls?
No. SOS was chosen for its simple, distinctive pattern, and readings like “Save Our Souls” and “Save Our Ship” came later.
How do I write SOS in Morse code?
As a prosign it is ...---... with no spaces. You will often see ... --- ... with letter spaces; people understand it, but that spells three letters, and the signal itself is sent with the elements run together.
How long should a dah be in SOS?
Three times as long as a dit. At 12 WPM that is 100 ms for each dit and 300 ms for each dah, with 100 ms of silence between elements.