Flags at sea are not decoration. They are communication — a system developed over centuries because ships at sea needed to exchange information at distances where voices, and later even radio, could not reach. The International Code of Signals was formalized in the mid-nineteenth century and remains in use, largely unchanged in structure, as a backup to radio and as a formal vocabulary for specific situations where a flag signal is still the correct answer.
Beyond the code itself, flag etiquette governs how sailors identify themselves, show respect for the countries they visit, display club membership, and signal their navigation or operational status. Getting these right signals competence and respect. Getting them wrong — particularly the courtesy flag — signals exactly the opposite.
The International Code of Signals: Key Flags
The full International Code uses 26 letter flags, 10 numeral pennants, 3 substitutes, and an answering pennant — 40 signals total. Each letter flag has a single-letter meaning when flown alone; combinations of two or three flags produce hundreds of additional meanings. In practice, most sailors will only ever encounter a handful of single-letter signals, but knowing the principal ones is basic competence.
Alpha (A) — Blue and white swallowtail means "I have a diver below; keep well clear at slow speed." This is the flag that matters most in crowded anchorages and near dive boats. When you see Alpha flying from a vessel, give it room. The diver below cannot hear you approaching.
Bravo (B) — Red, solid means "I am taking in, discharging, or carrying dangerous goods." Fuel docks may fly Bravo. Give it wide clearance and suppress the instinct to come alongside for a chat.
Oscar (O) — Red and yellow diagonal halves means "Man overboard." If you see Oscar flying from a vessel and hear a horn signal, the situation is active. This flag is rarely flown because most man-overboard situations are handled by radio; seeing it is a genuine emergency signal.
Quebec (Q) — Yellow, solid is the flag sailors encounter most often in foreign waters. It means "My vessel is healthy and I request free pratique" — the international term for customs and health clearance to enter a foreign port. You hoist Quebec when you enter foreign territorial waters and keep it flying until customs officials have cleared you. Failing to fly Quebec on arrival in a country is technically an offense, and customs officials in some countries will note it. Leaving Quebec up after clearance is incorrect — it signals you haven't been processed when you have been.
November (N) combined with Charlie (C) — the combination NC — is the international distress signal. It means "I am in distress and require immediate assistance." In the age of EPIRBs and DSC radio, you're unlikely to use it, but knowing it exists matters.
Lima (L) — Black and yellow quarters means "You should stop your vessel instantly." In practice this is used by port authorities and patrol vessels; seeing it from a moving official vessel means stop and await instructions.
Uniform (U) — Red and white quarters means "You are standing into danger." If you see this from a vessel near a shoal or obstruction, take it seriously.
The Courtesy Flag: Genuine Respect, Not Optional
When you sail into a foreign country, you fly the national flag of that country at the starboard spreader. This is the courtesy flag, and its history is straightforward: arriving ships traditionally lowered their weapons and displayed the host nation's colors as a signal of peaceful intent. The practice codified into maritime law and etiquette over centuries, and the underlying meaning hasn't changed: you are a guest in someone else's sovereign territory and you are saying so visually.
The courtesy flag goes at the starboard spreader — not at the masthead, not on the backstay, not on the port spreader. The starboard spreader is the internationally recognized position. It should be a proper flag, not a small paper printout or a decal. The flag should be flown from the moment you enter territorial waters (typically 12 nautical miles offshore) until you depart them.
Some sailors treat the courtesy flag as optional or skip it when they consider themselves in a hurry. This is poor form that is noticed by the marina staff, customs officials, and local sailors. In some countries — Mexico is the most commonly cited example — arriving without a courtesy flag creates immediate complications with the officials you need to process your entry paperwork. Flying the correct courtesy flag costs nothing and communicates respect that has practical value beyond the etiquette consideration.
Obtain the correct courtesy flags before departure if you know your ports of call. A simple set for the Caribbean, Mexico, or the Bahamas costs very little and takes no space aboard. The flags for less common destinations can often be sourced at major provisioning ports in advance.
The Burgee: Club Membership at the Masthead
The burgee — a club flag, typically swallow-tailed and triangular — traditionally flies at the masthead on a sloop or at the fore-truck on a schooner or ketch. On a modern sloop with a wind instrument at the masthead, it commonly moves to the port spreader, which is its accepted alternative position.
The burgee declares club membership. This matters more than it sounds: in many harbors, displaying the burgee of a recognized yacht club entitles the vessel to reciprocal privileges — use of the guest dock, access to the club facilities, sometimes preferential treatment from marina staff. Yacht clubs formally recognize each other's burgees for this purpose, and the arrangement is taken seriously. Displaying a burgee you're not entitled to fly — because you're not a member, or because you're borrowing a boat from a member and haven't confirmed privileges — is a breach of etiquette that will be recognized by anyone who cares about such things.
The masthead or port spreader is the correct position for the burgee. The starboard spreader belongs exclusively to the courtesy flag when in foreign waters. Flying the burgee at the starboard spreader when a courtesy flag should be there, or flying both at the starboard spreader, is incorrect.
The Ensign: Country and Registration
The ensign is the national flag of your vessel — it identifies the country under which the vessel is registered and operated. In most cases this is straightforward: an American-documented vessel flies the US ensign at the stern. The ensign is flown when underway and when at anchor in a foreign port; at a domestic marina it is conventional to fly it from colors at 0800 until sundown.
In the United States, the distinction between the yacht ensign and the US national ensign has generated genuine confusion. The yacht ensign — the US flag with an eagle and thirteen stars in the canton in place of the standard stars — was historically authorized for American pleasure vessels in domestic waters. The US national ensign (the standard flag) is correct in all waters including foreign. Current USCG guidance is that the national ensign is always appropriate and the yacht ensign is an optional domestic alternative, not a requirement. If you're making foreign passages, the national ensign is the unambiguous choice.
The US Power Squadron has its own ensign — a blue field with stars and the letters USPS — which member vessels may fly in place of or in addition to the national ensign in domestic waters. This is a membership signal analogous to the burgee, not a navigation or registration flag.
The ensign flies at the stern on a sloop — from a flagstaff at the transom or from the backstay at the leech of the mainsail. On a ketch or yawl it traditionally flies from the mizzen gaff or peak; on a schooner from the main gaff. Getting the position right for your rig is part of getting the etiquette right.
Flags You Should Not Fly
The pirate flag — the Jolly Roger in any of its variants — does not belong on a pleasure vessel. This is not a humor failure; it's an etiquette note that needs saying because the flags appear regularly on charter boats and occasionally on cruising vessels whose owners seem to have absorbed the Captain Jack Sparrow aesthetic without considering that piracy is an active criminal enterprise in some of the waters they intend to visit and that customs officials in those areas do not find the flag amusing. In the Gulf of Aden, the Malacca Strait, or parts of the Caribbean and West Africa, arriving with a pirate flag displayed is an active problem.
Flying the burgee of a yacht club you are not a member of is incorrect under the etiquette system, even if the practical consequences are limited. It's analogous to wearing someone else's military decorations.
Flying another country's ensign when your vessel is registered elsewhere — not as a courtesy flag but as if it were your own ensign — is a more serious error that can create complications with customs and coast guard officials who are looking at your registration documents and your flag and finding a discrepancy.
Signal flags flown in the wrong sequence or in combination that produces an unintended meaning are not technically wrong in the etiquette sense, but they do announce unfamiliarity with the code to any vessel that might be reading them.
Racing Flags and Start Sequences
Offshore and inshore racing uses a flag sequence for starting that is standardized by World Sailing rules and is worth knowing even if you don't race, because knowing what the race committee is communicating tells you how to behave around a starting area. The five-minute sequence typically runs: warning signal (class flag up), preparatory signal (P flag or I/Z/U flag indicating rules in effect), one-minute signal (P flag down), start. Hearing signals without the visual reference is insufficient — learn to read the flags.
The black flag (B, but flown by the race committee in a specific racing context) means any boat over the line during the minute before the start is automatically disqualified without a hearing. The AP (answering pennant) flown with the class flag means the start is postponed. These signals matter if you're in the start area.