NAUTICAL FLAG
ALPHABET

Spelling a word with signal flags is the least interesting thing you can do with them. In the International Code of Signals, a single flag flown on its own is already a complete message, and the messages it can carry are the ones you would not want to have to spell.

A means "I have a diver down; keep well clear at slow speed." O means "Man overboard." U means "You are running into danger" (International Code of Signals, Pub. 102, National Geospatial-Intelligence Agency).

Key Takeaways

• A full set is twenty-six alphabetical flags, ten numeral pennants, three substitutes, and the answering pennant.
• Single-letter signals are reserved for meanings that are "very urgent, important, or of very common use".
• The Code's governing rule is that each signal has a complete meaning, so nothing has to be assembled from a vocabulary.
• A ship carries one set of flags, so repeating a letter needs a substitute pennant rather than a second flag.
• The first international code, published in 1857, held 70,000 signals built from just eighteen flags.

What do the nautical signal flags mean on their own?

Almost every letter has a fixed meaning, chosen because it is the sort of thing a ship needs to say to a stranger quickly. The Code reserves single-letter signals for "significations which are very urgent, important, or of very common use" (Chapter 1, Section 1).

Here is the whole set, from the Code's own table (Chapter 1, Section 10).

Flag	Meaning in the Code
A	"I have a diver down; keep well clear at slow speed."
B	"I am taking in, or discharging, or carrying dangerous goods."
C	"Yes" (affirmative).
D	"Keep clear of me; I am maneuvering with difficulty."
E	"I am altering my course to starboard."
F	"I am disabled; communicate with me."
G	"I require a pilot." Among fishing vessels, "I am hauling nets".
H	"I have a pilot on board."
I	"I am altering my course to port."
J	"I am on fire and have dangerous cargo on board: keep well clear of me", or "I am leaking dangerous cargo".
K	"I wish to communicate with you."
L	"You should stop your vessel instantly."
M	"My vessel is stopped and making no way through the water."
N	"No" (negative).
O	"Man overboard."
P	In harbour, "All persons should report on board as the vessel is about to proceed to sea". At sea, among fishing vessels, "My nets have come fast upon an obstruction".
Q	"My vessel is 'healthy' and I request free pratique."
R	No single-letter meaning. See below.
S	"I am operating astern propulsion."
T	"Keep clear of me; I am engaged in pair trawling."
U	"You are running into danger."
V	"I require assistance."
W	"I require medical assistance."
X	"Stop carrying out your intentions and watch for my signals."
Y	"I am dragging my anchor."
Z	"I require a tug." Among fishing vessels, "I am shooting nets".

R is the hole in the alphabet. It is the one letter the Code's single-letter table skips. R does appear elsewhere, in the procedure signals, where it means "Received" or "I have received your last signal" (Chapter 1, Section 10).

The tempting explanation is that R was too busy to be a message as well. That explanation does not survive contact with the Code. C, N and K all hold down both jobs: each is a procedure signal, and each also carries a single-letter signification in the table above. The Code even prints a note (Chapter 1, Section 10) that "the procedure signals 'C', 'N', 'NO', and 'RQ' cannot be used in conjunction with single-letter signals", which only makes sense because those letters have single-letter meanings to collide with. R is simply left blank, and the Code does not say why.

Three letters change meaning depending on who is flying them or where. G and Z both switch among fishing vessels working close together, to "I am hauling nets" and "I am shooting nets" respectively. P switches by location: the harbour meaning is the recall of everyone ashore, but at sea a fishing vessel may use it for "My nets have come fast upon an obstruction".

Why does the code work in whole meanings rather than words?

It exists for the moment when nobody on either bridge speaks the other's language.

The Code states its purpose directly (Chapter 1, Section 1): "to provide ways and means of communication in situations related essentially to safety of navigation and persons, especially when language difficulties arise."

That leads to the design rule that shapes everything else. "The Code follows the basic principle that each signal should have a complete meaning" (Chapter 1, Section 1). An earlier version included a vocabulary method, where signals stood for individual words that you then assembled. The 1969 revision threw it out, and the Preface says dropping it and the Geographical Section together made it possible "to reduce considerably the volume of the Code and achieve simplicity". It also removes a whole class of mistake: with no assembly step, there is no assembly step to get wrong.

Beyond the single letters there are two-letter signals for the General Signal Code and three-letter signals beginning with M for the Medical Signal Code. Numerals can be added as complements to vary a meaning (Chapter 1, Section 1): "IN" is "I require a diver", and "IN 1" is "I require a diver to clear propeller".

Why can a ship not hoist the same signal flag twice?

Because it only has one of each, and this is the detail that gives the system its strangest piece of machinery.

A signal like "VV" would need two V flags. The Code's answer is the substitute, and it explains the reason without embarrassment: substitutes exist (Chapter 1, Section 5) "to enable the same signal flag, either alphabetical flag or numeral pennant, to be repeated one or more times in the same group, in case only one set of flags is carried on board."

The rule is positional. "The first substitute always repeats the uppermost signal flag of that class of flags which immediately precedes the substitute." So "VV" is hoisted as V, then the first substitute. The number 1100 becomes 1, first substitute, 0, third substitute.

There is a limit built in: "No substitute can ever be used more than once in the same group." Three substitutes, each usable once, is the whole redundancy budget. It is a compression scheme forced on the designers by the size of a flag locker.

How do you know your signal was understood?

By watching how high the other ship flies one particular pennant.

The Code defines two heights (Chapter 1, Section 2). "A hoist or signal is said to be at the dip when it is hoisted about half of the full extent of the halyards. A hoist or signal is said to be close up when it is hoisted to the full extent of the halyards."

The receiving station hoists the answering pennant at the dip as soon as it sees your hoist, and close up as soon as it understands it. When you haul your hoist down, theirs drops back to the dip, ready for the next one.

That is an acknowledgement protocol with two distinct states, seen at a distance, requiring no shared language and no equipment beyond a rope. Sighted-but-not-understood is a different signal from understood, and the difference is a few metres of halyard.

If the receiver can read the flags but cannot make sense of them, there are signals for that too (Chapter 1, Section 5): "ZL" means "Your signal has been received but not understood", and "ZQ" means "Your signal appears incorrectly coded. You should check and repeat the whole".

The most serious hoist in the book is two flags. NC is the International Code signal of distress, and the company it keeps says everything about its standing: the Code lists it as one of fourteen distress signals prescribed by the collision regulations, in the same numbered list as SOS in Morse and the spoken word MAYDAY (Chapter 4, Section 1). A gun fired at intervals of about a minute is on that list. So is a rocket throwing red stars, and so is slowly raising and lowering your arms outstretched to each side.

The Code then closes the obvious loophole. Using any of those signals for anything other than indicating distress "is prohibited", and so is using anything that might be confused with one.

Where did the international code come from?

From a British committee in 1855, and it was enormous.

The idea it was built on is older and has a named author. In his Telegraphic Signals; or Marine Vocabulary, published by T. Egerton in 1803, the naval officer Sir Home Riggs Popham set out a book of words and sentences numbered so that flags could send the number instead of the letters. His reason for writing it, given in the introduction, is the reason the whole discipline exists: "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).

Popham also saw 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. A numbered book is limited only by how long the book is, and half a century later a committee took him at his word.

The Code's own Preface tells the rest. The first international code "was drafted in 1855 by a Committee set up by the British Board of Trade. It contained 70,000 signals using eighteen flags and was published by the British Board of Trade in 1857 in two parts". Eighteen flags, seventy thousand meanings, and most seafaring nations adopted it.

What followed is a list of things going wrong and being fixed:

Year	What happened
1855 to 1857	First international code drafted and published, 70,000 signals from eighteen flags
1887 to 1897	Revised after an international conference in Washington in 1889
After 1918	The 1897 edition "did not stand the test of World War I"
1930 to 1932	New edition in seven languages, adopted at Madrid in 1932
1965	Present Code adopted by the Fourth Assembly of IMCO, in nine languages
1 April 1969	The 1969 edition takes effect

Russian and Greek were the two languages added in the modern revision, joining English, French, Italian, German, Japanese, Spanish and Norwegian. The Norwegian entry is there because the Preface records that the Scandinavian governments were asked in 1927 to pick one language between them, and that is the one they chose.

Why keep signal flags when every ship has radio?

Because the Code was never only about flags, and the flags solve a problem radio does not.

The same book covers flag signalling, flashing light, sound, voice over a loud hailer, radiotelegraphy, radiotelephony, and Morse by hand flags or arms (Chapter 1, Section 3). One set of meanings, seven ways to send them. That last method is worth naming carefully: it is Morse sent by waving, and it is not the same system as flag semaphore, which encodes letters in arm angles rather than in dots and dashes. The letter-spelling tables in the Code are the same phonetic alphabet you hear on the radio.

A hoist, though, is addressed by geometry. Fly it without an identity signal and the Code says it "will be understood that the signal is addressed to all stations within visual signaling distance" (Chapter 1, Section 5). No channel, no call sign, no acknowledgement round trip: everyone who can see you has been told, and everyone who cannot has not.

For a diver in the water under your hull, that is exactly the right broadcast domain. The same human limit sets the standard rate for flashing light, which the Code puts at forty letters per minute (Chapter 1, Section 3) rather than anything faster. These are systems built to be read by a person, in weather, at distance, and getting them wrong drowns somebody.

Compare it with Morse code, which encodes the letters and leaves the meaning to you, or with the rest of the codes and ciphers, most of which were designed to stop people reading them rather than to make sure they could.

Frequently asked questions

What does a single signal flag mean?

Twenty-five of the twenty-six alphabetical flags have a fixed meaning on their own, reserved by the Code for significations that are very urgent, important or in very common use. A means a diver is down, O means man overboard, U means you are running into danger, and V means I require assistance. R is the exception: the Code's single-letter table simply skips it. R does serve as the procedure signal for "Received", but that alone does not explain the gap, because C, N and K hold both roles at once. The Code does not say why.

How many flags are in a signal set?

Twenty-six alphabetical flags, ten numeral pennants, three substitutes and the answering pennant. The substitutes exist because a ship carries only one of each flag and some signals repeat a letter.

What is the answering pennant for?

Acknowledgement. The receiving station raises it half way up, at the dip, when it sees a hoist, and all the way, close up, when it has understood it. The transmitting station also hoists it alone after the final group to show the message is complete.

What is the yellow Q flag?

Q on its own means "My vessel is 'healthy' and I request free pratique", pratique being permission to enter port and deal with people ashore. It is a health declaration, which is why it is associated with quarantine.

Who publishes the International Code of Signals?

The present Code was adopted by the Fourth Assembly of the Inter-Governmental Maritime Consultative Organization in 1965 and took effect on 1 April 1969. The United States edition is published as Pub. 102 by the National Geospatial-Intelligence Agency.

Spell something with the flags above, then look up what each of those letters would mean flown on its own. Four of them, A, D, J and T, are literally the words "keep clear" or "keep well clear", and several more are warnings in everything but wording. When you are done, flag semaphore is a separate arm-powered code with its own alphabet, and the daily flag game tests whether any of this stuck.

Sources

• Sir Home Riggs Popham, Telegraphic Signals; or Marine Vocabulary, T. Egerton, Military Library, near Whitehall, 1803 - retrieved 3 August 2026, https://archive.org/details/bub_gb_qxZEAAAAYAAJ
• National Geospatial-Intelligence Agency, Pub. 102: International Code of Signals, United States Edition, 1969 (Revised 2020) - retrieved 3 August 2026, https://msi.nga.mil/Publications/ICOS