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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 (INTERCO). ¹

There are various methods that the flags can be used as signals:

• each flag spells out an alphabetic message, letter by letter.
• individual flags have specific and standard meanings²; 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, 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" (for more details see Race Signals).

NATO uses the same flags, with a few unique to warships, alone or in short sets to communicate various unclassified messages. The NATO usage generally differs from the International meanings, and therefore warships will fly the Code/Answer flag above the signal to indicate it should be read using the International meaning.

Letters

A I have a diver down; keep well clear at slow speed. With three numerals, azimuth or bearing.	B I am taking in, or discharging, or carrying dangerous goods. (Originally used by the Royal Navy specifically for military explosives.)	C Affirmative.* ** With three numerals, course in degrees magnetic.	D Keep clear of me; I am maneuvering with difficulty. With two, four, or six numerals, date.
E I am altering my course to starboard.**	F I am disabled; communicate with me.	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.)	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. 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.	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.)
M My vessel is stopped and making no way through the water.**	N Negative.*	O Man overboard.** (often attached to the man overboard pole on boats). With a sinister hoist, the semaphore flag.	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".
Q My vessel is "healthy" and I request free pratique.	R The way is off my ship. With one or more numerals, distance in nautical miles.	S I am operating astern propulsion.** With one or more numerals, speed in knots.	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.)
U You are running into danger.	V I require assistance. With one or more numerals, speed in kilometres per hour.	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. 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.

** Also signallable on a ship's whistle using Morse code. See vessel emergency codes and International Regulations for Preventing Collisions at Sea.

Numbers

Type of flag	0	1	2	3	4	5	6	7	8	9
Allied flags (outdated)
Pennants (modern)

Other flags

First substitute	Second substitute	Third substitute	Fourth substitute
Allied signals
Code/answer (ANS)	Preparative (PREP)	Question (INT)	Negation (NEGAT)	Designation (DESIG)
Course Pennant (CORPEN)	Turn (TURN)	Screen (SCREEN)	Speed (SPEED)	Station (STATION)
Port (PORT)	Starboard (STBD)	Formation (FORM)	Division (DIV)	Squadron (SQUAD)
Group (FLOT)	Subdivision (SUBDIV)	Emergency (EMERG)

See also

• Aldis lamp
• Character encoding
• England expects that every man will do his duty
• Flag semaphore
• Flag signals
• Heliograph
• NATO phonetic alphabet
• Signal lamp

References

(1995) International Code of Signals. International Maritime Organisation. 

External links

Wikimedia Commons has media related to:

International Code of Signals

• Brief history of naval signals
• Freeware to aid memorizing the flags from The Navy Patuxent Sailing Club
• La flag-alfabeto - signal flags used for the Esperanto language - the flags for the Esperanto letters with diacritical marks have the lighter color in the normal flag replaced with light green, which is not used in any normal flag.
• Signalman manual