International Code of Signals
The system of international maritime signal flags is a way of representing individual letters of the alphabet in signals to or from ships. It is a component of the International Code of Signals. Nautical signal flags can be used in various methods:
- each flag spells out a letter of an alphabetic message.
- individual signal flags have specific and standard meanings; for example, diving support vessels raise the "A flag" indicating their inability to move from their current location because they have a diver underwater.
- one or more flags form a code word whose meaning can be looked up in a code book held by both parties. An example is the Popham numeric code used at the Battle of Trafalgar.
- in yacht racing and dinghy racing, nautical signal flags have other meanings; for example, the P flag is used as the "preparatory" flag to indicate an imminent start, and the S flag means "shortened course".
- Note: NATO uses an extra set of flags to communicate specific types of messages.
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| Letter | Meaning |
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A - I have a diver down; keep well clear at slow speed. With three numerals, azimuth or bearing. |
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B - I am taking in, or discharging, or carrying dangerous goods. (Originally used by the Royal Navy specifically for military explosives.) |
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C - Affirmative. With three numerals, course. |
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D - Keep clear of me; I am maneuvering with difficulty. With two, four, or six numerals, date. |
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E - I am altering my course to starboard. |
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F - I am disabled; communicate with me. |
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G - I require a pilot. When made by fishing vessels operating in close proximity on the fishing grounds it means: "I am hauling nets". With four or five numerals, longitude. (The last two numerals denote minutes and the rest degrees.) |
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H - I have a pilot on board. |
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I - I am altering my course to port. |
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J - I am on fire and have dangerous cargo on board: keep well clear of me, or I am leaking dangerous cargo. |
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K - I wish to communicate with you. With one numeral, I wish to communicate with you by; 1) Morse signalling by hand-flags or arms; 2) Loud hailer (megaphone); 3) Morse signalling lamp; 4) Sound signals. |
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L - In harbor: The ship is under Quarantine. At sea: You should stop your vessel instantly. With four numerals, latitude. (The first two denote degrees and the rest minutes.) |
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M - My vessel is stopped and making no way through the water. |
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N - Negative. |
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O - Man overboard. (often attached to the man overboard pole on boats). |
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P - The Blue Peter. In harbor: All persons should report on board as the vessel is about to proceed to sea. At sea: It may be used by fishing vessels to mean: "My nets have come fast upon an obstruction". |
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Q - My vessel is "healthy" and I request free pratique. |
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R - The way is off my ship. With one or more numerals, distance in nautical miles. |
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S - I am operating astern propulsion. With one or more numerals, speed in knots. |
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T - Keep clear of me; I am engaged in pair trawling. With four numerals, local time. (The first two denote hours and the rest minutes.) |
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U - You are running into danger. |
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V - I require assistance. With one or more numerals, speed in kilometres per hour. |
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W - I require medical assistance. |
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X - Stop carrying out your intentions and watch for my signals. |
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Y - I am dragging my anchor. |
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Z - I require a tug. When made by fishing vessels operating in close proximity on the fishing grounds it means: "I am shooting nets". With one or more numerals, time (UTC). (The first two denote hours and the rest minutes.) (Origin of the phrase 'Zulu Time'.) |
* N and C together (No and Yes) is used as a distress signal.
Numbers
| Type of flag | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| NATO flags |  |
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| Pennants |  |
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Other Flags
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| Prompt | First substitute | Second substitute | Third substitute | Fourth substitute |
Storm Signal Flags
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| Hurricane | Gale |
NATO Signals
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| Code/answer (ANS) | Preparation (PREP) | Question (INT) | Negation (NEGAT) | Designation (DESIG) |
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| Alter course (CORPEN) | Turn (TURN) | Screen (SCREEN) | Speed (SPEED) | Station (STATION) |
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| Port (PORT) | Starboard (STBD) | Formation (FORM) | Division (DIV) | Squadron (SQUAD) |
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| Group (FLOT) | Subdivision (SUBDIV) | Emergency (EMERG) |