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CODES · SIGNALLING

FLAG SEMAPHORE
ALPHABET

Every letter is a pair of flag positions, held out from the body at one of eight angles. See the full A-Z chart, then type your own text to watch it flagged out letter by letter.

The full flag semaphore alphabet, A to Z:

A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
IN SEMAPHORE
O
N
E
M
O
R
E
C
L
I
C
K
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A-Z · 0-9 · PUNCTUATION

THE LONG ANSWER ◢

Flag semaphore spells a letter with the angle of two arms. Each arm is held out in one of eight directions, and the pair of angles is the character. The flags carry no part of the letter: they are there to make the arms visible from further away.

The system covers 26 letters, the numerals and a handful of control signals, and it is read while the flags are still. Motion is just travel between letters.

Key Takeaways

  • Each arm takes one of eight positions. The letter is the combination, read when the arms stop moving.
  • It descends from Claude Chappe's tower telegraph, which grew to 556 stations across 4,800 km of France.
  • Chappe's towers could form 196 symbols, from two seven-position arms and a four-angle crossbar.
  • The handheld flag version arrived in 1866, after the two-arm masts of Sir Home Riggs Popham and Charles Pasley.
  • At sea the flags are red and yellow; on land they are white and blue.

How does flag semaphore work?

Two flags, one in each hand, and eight places to put each arm.

The signaller holds a pole in each hand and extends each arm in one of eight possible directions, giving a set of about 30 characters that covers the alphabet, the numerals and the signals that manage the conversation (Wikipedia, Flag semaphore).

Eight directions for each of two arms sounds like it should yield far more than thirty, and it does. The system does not use every possible pairing: combinations where the arms would overlap, or sit too close to tell apart at distance, are left out. What survives is a set chosen for legibility rather than for capacity.

Nor does thirty leave room for ten digits on top of 26 letters. It does not need to. The numerals reuse the first ten letter positions, with J sitting in the same run as the way back out. That is what the numeral sign exists to announce: send it, and A means one until J says otherwise.

The important detail for reading it is that a character is a held position, not a movement. The arms sweep from one letter to the next, and everything during the sweep is noise. A receiver watches for the pause.

The chart above groups the letters accordingly, rather than in any order that would look tidy. Look along one of its groups and you will see one arm held still while the other works round, so consecutive letters are a single arm-move apart and a signaller can run through a group without hunting. Crossing from one group to the next is where both arms have to move.

What are the special signals?

The ones that stop a message becoming gibberish (all from Wikipedia, Flag semaphore).

  • Attention asks for permission to start.
  • Numerals warns that what follows is digits rather than letters.
  • J, confusingly, doubles as the sign meaning letters to follow, switching back out of numeral mode.
  • Cancel or annul means disregard what I just sent.
  • Error is the letter E sent eight times in a row, which is unmistakable precisely because no real word does that.

Put together, a short exchange runs like this. Attention, and you wait to be seen. Then the letters, one held position at a time. Get one wrong and you send E eight times, which is the apology, and start the word again. Need to send a number, and you send the numeral sign first, then letters that are read as digits until J puts you back into words. Change your mind entirely and cancel wipes the whole thing.

The error signal is the one worth pausing on. There is no back-channel in semaphore, no acknowledgement byte, no retransmit request. The only way to unsay something is to send a pattern so obviously wrong that a human reader cannot mistake it for content.

What colour are semaphore flags?

It depends which side of the waterline you are standing on (Wikipedia, Flag semaphore).

At sea the flags are red and yellow, the colours of the Oscar flag in the maritime signal set. On land they are white and blue, matching the Papa flag. Whatever the reasoning was, the two pairs belong to different settings: Oscar's red and yellow at sea, Papa's white and blue ashore.

The flag is a magnifier, not a symbol. Semaphore can be sent with rods, disks, paddles, or with nothing but gloved hands, and it still works at short range. How far it carries depends on the optics at the far end, the light, and how big the flags are, so there is no single figure for it.

What problem was any of this solving?

Shouting, and how badly it works at sea.

The best answer comes from Sir Home Riggs Popham, the naval officer whose signalling work stands directly behind the flag alphabet. In the introduction to his 1803 Telegraphic Signals; or Marine Vocabulary, he sets out the case plainly: "Instances of the impossibility of making communications by trumpet in heavy gales of wind, when it is of the last importance to convey intelligence, have happened so frequently, that it is unnecessary to recapitulate them" (Popham, 1803).

That is the whole brief. Two ships, a gale, something urgent, and a speaking trumpet that cannot cross the gap.

Popham's answer was a numbered vocabulary. His book listed words and sentences against numbers, and the flags sent the number rather than the letters. He was explicit that this was an addition rather than a replacement: "It is by no means intended to interfere with the established signals, as a single signal is certainly the most efficient for military evolutions." The established signals said tack now. His vocabulary let you say things nobody had thought to allocate a signal for.

He also saw exactly how far it could scale. Number the words of "an Entick or other small dictionary", he suggested, and the vocabulary would reach as far as the dictionary itself. And he noticed what it implied: "This Vocabulary may be also used as a cipher, either in the common or secret keys."

Popham later applied the same thinking to a two-arm mast, a single fixed 30-foot pole with two movable 8-foot arms, and the Admiralty line from London to Portsmouth ran from 1822 until 1847. Charles Pasley designed a separate two-arm system in which each arm took eight positions, which is the number the handheld alphabet uses (Wikipedia, Optical telegraph).

Handheld flag semaphore arrived in 1866, as a portable version of the same idea with a human body in place of the mast (Wikipedia, Flag semaphore).

Laid side by side, the three generations shrink in every dimension except usefulness:

Generation Chappe tower Popham and Pasley masts Handheld flags
In use 1791 onwards London to Portsmouth, 1822 to 1847 1866 onwards
Moving parts Two arms plus a crossbar Two arms on a fixed pole Two human arms
Positions per arm Seven, plus four crossbar angles Eight on Pasley's Eight
Characters 196 symbols indexing a word book A numbered vocabulary About 30, letters and control
Needs A staffed building every few kilometres A permanent mast A person

What were the towers, and why did they stop?

They were the land version, and they were enormous.

Claude Chappe's optical telegraph made its first public demonstration on 2 March 1791, between Brûlon and Parcé, over 16 kilometres (Wikipedia, Optical telegraph). Each station was a tower carrying a jointed wooden indicator, and an operator with a telescope watched the previous tower and copied whatever it was showing.

Two movable arms two metres long, each able to show seven positions, hung from a 4.6-metre crossbar that could sit at four different angles. Seven by seven by four is 196 symbols, far more than an alphabet needs, because those symbols indexed a book of words rather than a set of letters (Wikipedia, Semaphore line). Chappe got there first, and Popham would later set out the same principle in his own words.

The speed is the part that still sounds wrong. On the Paris to Lille line, 230 kilometres, "the first symbol of a message to Lille would pass through 15 stations in only nine minutes" (Wikipedia, Semaphore line). A full message of 36 symbols took around 32 minutes end to end. At its peak the network ran to 556 stations covering 4,800 kilometres, so France had a working national telecommunications system before it had a working electric one.

What killed it was weather and payroll. Towers cannot work in fog or darkness and need a staffed building every few kilometres. A decision was taken in 1846 to replace the network with the Foy-Breguet electrical telegraph, though the changeover took years, and one of the last messages it carried was the report of the fall of Sebastopol in 1855 (Wikipedia, Optical telegraph).

The handheld version outlived the towers for the obvious reason. It needs no building, no fuel and no establishment. It needs a person.

Is flag semaphore still used?

Yes, and for a reason that had not been invented when the system was (Wikipedia, Flag semaphore).

It remains in use during underway replenishment at sea, where two ships steam side by side a few dozen metres apart, and it is accepted for emergency communication in daylight or at night with lighted wands instead of flags.

One reason often given is that it emits nothing. Radio can be intercepted, direction-found and jammed. Two people waving flags at each other put no signal on the air at all, so there is nothing to detect beyond the stretch of water between them. A system built because electricity did not exist yet is now useful because sometimes you would rather not use it. The maritime signal flags hoisted on a halyard are quiet for the same reason, and the daily flag game is about the only place most people meet either set now.

Compare that with Morse code, which solved the same problem with time instead of position and turned out to be far easier to automate. Semaphore never got a machine. It is still one person, watching another person, waiting for them to hold still.

Frequently asked questions

How many positions can each semaphore arm take?

Eight. The letter is the combination of both arms, held still. The full set covers 26 letters, the numerals and control signals such as attention, cancel and error.

What does waving both flags mean?

Nothing on its own. Semaphore characters are read from held positions, and the movement between them carries no meaning. The nearest thing to an exception is the error signal, which is the letter E held eight times over rather than any kind of wave.

Why are semaphore flags red and yellow?

Those are the sea colours, matching the Oscar flag in the maritime signal set. Ashore the flags are white and blue, matching the Papa flag. The Code sets out which pair belongs where; it does not record why those colours were picked.

Who invented semaphore?

The tower version was Claude Chappe's, first demonstrated publicly on 2 March 1791. The handheld flag version dates to 1866 and descends from the two-arm masts of Sir Home Riggs Popham and Charles Pasley.

Do navies still use flag semaphore?

Yes. It is used during underway replenishment and is accepted for emergency communication, in daylight with flags and at night with lighted wands. One reason often given for keeping it is that it puts no signal on the air, so nothing can be picked up beyond the line of sight between the two ships.

Send yourself a word with the chart above, then try holding the positions and see how quickly your arms start arguing with each other. When you are done, the maritime flag alphabet is the other half of the same conversation, and the rest of the codes are waiting.

Sources