Nautical flags are the 40 signal flags of the International Code of Signals: 26 letter flags, 10 numeral pennants, 3 substitutes and 1 answering pennant. Each letter flag spells a letter, and most also carry a fixed message when flown alone, such as "man overboard" or "I require a pilot." Combined in groups of two or three, they send hundreds of standard messages that any crew can read, whatever language they speak.
If you have ever seen a string of colourful flags on a moored ship, a lone blue and white swallowtail over a dive boat, or a yellow flag on a freighter arriving in port, you have seen this system at work. This guide covers every flag in the set, what each one means, how the combinations are built, and where you will still see them flying today.
What Nautical Flags Are and Who Sets the Rules
The flags belong to a single rulebook: the International Code of Signals, maintained by the International Maritime Organization. In the United States it is published by the National Geospatial-Intelligence Agency as Pub. 102, a free download that covers flags, flashing light, sound, radio and hand signals in one volume.
The code exists for one purpose: to let ships communicate about safety of navigation and people, especially when the crews do not share a language. A Greek master and a Korean pilot do not need a common tongue to agree that a ship has a pilot aboard. They only need the same flag, hoisted the same way.
That is also why the code is built on meanings rather than words. A single flag or a short group stands for a complete sentence. The receiving ship looks up the group in its own copy of the code, printed in its own language, and reads the full message.
Nautical signal flags are only one channel of the code. The same letter groups can be sent by Morse lamp, by sound signal, or spoken over the radio. The flags are simply the version you can see from a distance, in daylight, without any power at all.
A Short History of the International Code of Signals
The first international code was drafted in 1855 by a committee set up by the British Board of Trade. It contained 70,000 signals built from eighteen flags, and the Board of Trade published it in 1857 in two parts: one with universal signals for all nations, and one with British signals only.
That first version went through several revisions, and one edition, distributed to the maritime powers of the time, did not stand the test of the First World War.
The modern code was adopted by the Fourth Assembly of the Inter-Governmental Maritime Consultative Organization, the body now called the IMO, in 1965. Its United States edition took effect on 1 April 1969. Pub. 102 is still officially "the 1969 edition," with the latest revision dated 2020.
The 1969 code brought one big change in philosophy. Every signal has a complete meaning, so a crew never has to assemble a sentence from fragments. It also merged the separate radio and visual volumes into a single book suited to every method of signalling.
The Complete Set: 40 Flags in Four Groups
Pub. 102 defines a full set of signal flags as twenty-six alphabetical flags, ten numeral pennants, three substitutes and the answering pennant. Every flag in that set falls into one of four groups.
The letter flags are mostly rectangular, with two exceptions: A and B are swallowtailed, cut with a notch in the fly end. Each flag is named with the phonetic alphabet, Alfa, Bravo, Charlie and so on, which is why sailors talk about "flying Bravo" rather than "flying B."
The numeral pennants are tapered and carry the digits 0 to 9. Using a pennant shape for numbers means a reader can tell a digit from a letter at a glance, even before the colours are clear.
The three substitutes, also called repeaters, are triangular pennants that repeat a flag already in the same hoist. They let a ship carrying only one set of flags send a group with the same letter or number twice.
The answering pennant, sometimes called the code pennant, has red and white vertical stripes. It acknowledges a signal and also serves as a decimal point when sending numbers.
The whole set uses only five colours: red, blue, yellow, black and white.
Nautical Alphabet Flags A to I
The nautical alphabet flags are the core of the system. Here are the first nine, with their design and the meaning each carries when flown on its own, as listed in the code.
| Letter | Name | Design | Meaning flown alone |
|---|---|---|---|
| A | Alfa | White and blue, swallowtailed | I have a diver down; keep well clear at slow speed |
| B | Bravo | Solid red, swallowtailed | I am taking in, discharging or carrying dangerous goods |
| C | Charlie | Blue, white, red, white, blue horizontal stripes | Affirmative (yes) |
| D | Delta | Yellow, blue, yellow horizontal bands | Keep clear of me; I am maneuvering with difficulty |
| E | Echo | Blue over red | I am altering my course to starboard |
| F | Foxtrot | White with a red diamond | I am disabled; communicate with me |
| G | Golf | Six vertical yellow and blue stripes | I require a pilot |
| H | Hotel | White and red, split vertically | I have a pilot on board |
| I | India | Yellow with a black disc | I am altering my course to port |
Two flags here carry extra meanings for fishing vessels. Golf, flown by a fishing vessel working close to others on the fishing grounds, means "I am hauling nets." Bravo was originally used by the Royal Navy specifically for military explosives before it widened to all dangerous goods.
Nautical Alphabet Flags J to R
The middle of the alphabet holds some of the most urgent single-flag signals in the code, including the one every sailor learns first: Oscar.
| Letter | Name | Design | Meaning flown alone |
|---|---|---|---|
| J | Juliett | Blue, white, blue horizontal bands | I am on fire and have dangerous cargo on board; keep well clear |
| K | Kilo | Yellow and blue, split vertically | I wish to communicate with you |
| L | Lima | Yellow and black quarters | You should stop your vessel immediately |
| M | Mike | Blue with a white diagonal cross | My vessel is stopped and making no way through the water |
| N | November | Blue and white checks | Negative (no) |
| O | Oscar | Red and yellow, split diagonally | Man overboard |
| P | Papa | Blue with a white square | In harbour: all persons report on board, the vessel is about to sail |
| Q | Quebec | Solid yellow | My vessel is "healthy" and I request free pratique |
| R | Romeo | Red with a yellow cross | No single-flag meaning in the general code |
Papa is the famous Blue Peter. At sea, a fishing vessel can fly it to say its nets have come fast upon an obstruction. Kilo also works with a number after it: K1 asks to communicate by hand flags or arms, K2 by loud hailer, K3 by Morse lamp, and K4 by sound signals.
Nautical Alphabet Flags S to Z
The last eight letters complete the set. Several of them are signals you hope never to need.
| Letter | Name | Design | Meaning flown alone |
|---|---|---|---|
| S | Sierra | White with a blue square | I am operating astern propulsion |
| T | Tango | Red, white, blue vertical bands | Keep clear of me; I am engaged in pair trawling |
| U | Uniform | Red and white quarters | You are running into danger |
| V | Victor | White with a red diagonal cross | I require assistance |
| W | Whiskey | Blue border, white band, red centre | I require medical assistance |
| X | X-ray | White with a blue cross | Stop carrying out your intentions and watch for my signals |
| Y | Yankee | Yellow and red diagonal stripes | I am dragging my anchor |
| Z | Zulu | Yellow, blue, red and black triangles | I require a tug |
Pub. 102 marks some of these single-letter signals with an asterisk. Those, when made by sound rather than flag, may only be used in line with the collision regulations, because a ship's whistle already carries meaning under those rules. You can read how those sound signals fit into the rules of the road in our guide to COLREGS.
The Single Flags Every Boater Should Know
Twenty-five single-flag meanings is a lot to memorise. In practice, a handful show up far more often than the rest, and knowing these covers most of what you will see on the water.
Oscar: man overboard
Oscar is the most widely known nautical flag. Its red and yellow diagonal halves are bright against both sea and sky, and many small boats attach a small Oscar to the man overboard pole thrown to a person in the water. If you see Oscar flying, assume someone is in the water and keep a sharp lookout.
Quebec: the yellow flag in port
The solid yellow flag was the quarantine flag before it became Quebec. The meaning today is close to the opposite of its old reputation. A ship flying Q is declaring that it is healthy and asking the port authorities for free pratique, the permission to enter port and land people and cargo. Yachts arriving in a new country often fly Q until customs and health clearance are complete.
Hotel and Golf: the pilot flags
Hotel, "I have a pilot on board," is one of the flags you are most likely to see on a large commercial ship entering or leaving port. Golf asks for a pilot. Together they mark the moment a local expert takes over navigation in tricky approaches.
Bravo: dangerous goods
A solid red swallowtail flying from a tanker or a ship at a fuel berth signals dangerous goods being loaded, discharged or carried. It is a cue for other vessels, and for anyone on the dock, to stay clear and keep open flames away.
Yankee, Victor and Whiskey: trouble at anchor or aboard
Yankee warns that a ship is dragging its anchor, a real hazard in a crowded anchorage during a blow. Victor asks for assistance and Whiskey for medical help. None of these mean the ship is sinking, but each asks nearby vessels to pay attention.
Delta, Juliett, Lima and Uniform: keep clear or stop
A group of flags exists purely to warn other ships off. Delta says a vessel is maneuvering with difficulty, and Juliett that it is on fire with dangerous cargo aboard, so both ask you to keep well clear. Lima orders you to stop your vessel immediately, Uniform tells you that you are running into danger, and X-ray tells you to stop what you are doing and watch for further signals. If one of these is aimed at you, act first and ask questions over the radio afterwards.
Numeral Pennants, Substitutes and the Answering Pennant
Numbers have their own flags so that a position, a course or a time can be sent without spelling every digit as a word.
| Pennant | Design | Pennant | Design |
|---|---|---|---|
| 0 | Yellow with a red vertical stripe | 5 | Yellow and blue halves |
| 1 | White with a red disc | 6 | Black over white |
| 2 | Blue with a white disc | 7 | Yellow over red |
| 3 | Red, white and blue vertical bands | 8 | White with a red cross |
| 4 | Red with a white cross | 9 | White, black, red and yellow quarters |
How the substitutes work
A ship that carries only one set of flags cannot fly the same letter twice. The substitutes solve that. Each substitute repeats a flag already in the hoist: the first substitute repeats the first flag of the same class, letters or numbers, counting from the top, the second repeats the second, and the third repeats the third.
So a group that needs two of the same letter in a row is sent as that letter with the first substitute below it. The reader sees the substitute, looks up to the flag it points to, and reads the letter twice.
The answering pennant
The answering pennant does two jobs. Hoisted by a receiving ship, it confirms that a signal has been seen and understood. Placed between numeral pennants, it works as a decimal point.
Navies add a third use. When a warship wants to talk to a merchant vessel in the international code, Pub. 102 says it hoists the code pennant in a conspicuous position and keeps it flying for the whole signal. NATO flag meanings differ from the international ones, so this pennant tells everyone to read the hoist with the civilian code book.
How Two- and Three-Flag Signals Work
Single flags cover urgent, common situations. Everything else uses groups. Two-letter groups form the general signal code, covering topics from towing and ice to navigation lights and aircraft. Three-letter groups starting with M form the medical signal code, used to describe symptoms and treatment when a ship has no doctor aboard.
A hoist is read from the top down. When several hoists fly at once, the code sets an order for reading them, so a long message can be split across halyards without losing its sense.
Letters with numbers
Some letters change meaning when followed by numerals. These are the main ones from Pub. 102:
- A with three numerals gives an azimuth or bearing
- C with three numerals gives a course
- D with two, four or six numerals gives a date
- G with four or five numerals gives a longitude, and L with four numerals a latitude
- R with one or more numerals gives a distance in nautical miles
- Z with four numerals gives the time in GMT
With those few rules, a crew can send a full position, course and time using flags alone, and any ship reading them will get the same numbers.
Distress and assistance groups
The code reserves short groups for the worst situations. NC is the international code signal of distress. AE means "I must abandon my vessel," CB means "I require immediate assistance," CB6 adds "I am on fire," and DX means "I am sinking." The standard reply is CP: "I am proceeding to your assistance."
Two more groups shape every conversation. CQ calls all stations in the vicinity, and YZ tells the receiver that the words that follow are in plain language rather than code. Adding N or NO after a signal turns it negative, and RQ turns it into a question, so CW RQ asks "Is boat or raft on board?"
Diver Down: Alpha Flag vs the Red and White Flag
Diving has two flags, and which one you need depends on where you are.
Under the international code, flag Alfa means "I have a diver down; keep well clear at slow speed." Outside North America it is the standard dive signal. A vessel engaged in diving operations that is restricted in its ability to manoeuvre, and too small to show the lights and shapes the collision rules require, must display a rigid replica of Alfa instead.
In the United States and Canada, divers mostly use a different design: red with a white diagonal stripe from the upper left to the lower right. The red and white flag was created in the early 1950s by Navy veteran Denzel "Doc" Dockery of Michigan, and its use is required by law or regulation in many US states and in Canada.
The practical rule is simple: when you see either flag, slow down and keep well clear. Both flags mark water where a diver may surface at any moment, often well away from the boat itself.
Signal Flags in Sailboat Racing
If you spend time near a yacht club, racing is where you will see nautical flags used most actively. The racing rules published by World Sailing borrow flags from the international set and give many of them new meanings for the race course.
A typical start runs on a five-minute sequence. The class flag goes up at five minutes as the warning signal. At four minutes the race committee hoists the preparatory signal, usually flag P, or I, Z, U or a black flag when a stricter start rule applies. The preparatory flag comes down at one minute, and the class flag comes down at the start.
Other racing signals change the day as it unfolds. AP, the answering pennant, postpones races not yet started; AP over A means no more racing today. N abandons races in progress. X signals an individual recall for a boat over the line early, while the first substitute calls a general recall of the whole fleet. S means the course has been shortened, and Y tells crews to wear a personal flotation device.
None of these racing meanings apply outside the race. A yacht flying P in a marina is not about to start a race, and a committee boat flying S is not operating astern propulsion. Context decides which code you are reading.
Dressing Ship: When Flags Mean Nothing at All
On national holidays, ship launches and special occasions, naval and passenger ships "dress overall," running a line of signal flags from the bow, up over the masts and down to the stern. It is the most colourful use of nautical flags, and the only one where the flags are meant to say nothing.
According to the flag historians at Flags of the World, navies prescribe a fixed order for dressing lines for two reasons: to make sure no hidden message, intentional or not, appears in the display, and to keep warships looking uniform. The order is also chosen to spread the colours evenly, so the yellow, red and blue flags do not bunch together.
Each navy has its own sequence. The French Navy calls the display "grand pavois" and builds it from the alphabet flags plus the first substitute. If you try to read a dressing line as a message, you will get gibberish, which is exactly the point.
Do Ships Still Use Signal Flags Today?
Yes, though less for conversation and more for status. Long messages now travel by VHF radio, satellite and email. Ships also broadcast their identity, position, speed and navigation status automatically through AIS, the system that lets anyone follow a vessel on a live map.
Some flag signals have kept their place because they work without power, without radio and across language barriers. You will still regularly see these hoists:
- H on ships with a harbour pilot aboard
- B on tankers and ships handling dangerous cargo
- Q on ships and yachts arriving from abroad
- A or the red and white diver flag over dive boats
- P on some vessels about to sail, and in every sailing race start
Navy and coast guard vessels also fly their call signs from signal halyards, typically on the port side, with outgoing messages on the starboard side. A photo of the US command ship Blue Ridge shows the idea: one hoist for a pilot aboard, one coded message and one call sign, all flying at once.
Flags and AIS complement each other. A flag tells you what a ship is doing right now in the harbour in front of you, while AIS tells you who the ship is and where it is heading. If you spot a ship flying Hotel and want to know where it is going, search its name in Primo Nautic to see its live position, speed, course and navigation status, then read the AI summary that explains the voyage in plain language.
How to Learn the Nautical Flags Alphabet
Learning 40 flags is easier than it sounds, because the code was designed to be learned by working sailors under pressure. A few habits make it stick.
Start with the flags that matter for safety rather than going from A to Z. Oscar, Alfa, Victor, Whiskey and the diver flag are the ones you may actually need to recognise and act on. Once those are solid, add the port and harbour flags, Hotel, Golf, Bravo and Quebec, which you will see most often.
Learn the colour logic next. All the flags use only five colours, and many are simple splits: halves, quarters, stripes or a single shape on a plain field. Saying the design out loud, "Kilo, yellow and blue halves," helps more than staring at a chart.
Use the phonetic names from day one. Radio procedure, flag signalling and the racing rules all use Alfa, Bravo and Charlie, so learning the flags and the phonetic alphabet together saves you doing the work twice.
Then put it into practice. Walk a marina or a working port and name every flag you see. If you want structured study, Primo Nautic hosts maritime courses on its website, including COLREGS and navigation, that put flag and sound signals in context with the rest of the rules. For the bigger picture of how ships talk to each other, from flags and Morse to VHF and satellite, see our overview of maritime communication methods.
Conclusion
Nautical flags are a complete language built into 40 pieces of coloured cloth. The International Code of Signals gives every one of the 26 letter flags a letter, and most of them a full sentence when flown alone, while the numeral pennants, substitutes and answering pennant handle numbers, repeats and replies.
The code dates back to 1857 and took its modern form in 1965, but its core idea has not changed: a ship should be able to say something important to any other ship, in any language, with nothing but a halyard and a set of flags. Radio and AIS now carry most messages at sea, yet the pilot flag, the dangerous goods flag, the yellow Q and the diver flags still fly every day.
If you remember only a few, make them Oscar for man overboard, Alfa and the red and white flag for divers, and Victor and Whiskey for a ship asking for help. Those are the flags where knowing the meaning can change what you do next.
