International maritime signal flags are various flags used to communicate with ships. The principal system of flags and associated codes is the International Code of Signals.^[1] Various navies have flag systems with additional flags and codes, and other flags are used in special uses, or have historical significance.^[2]

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

• A series of flags can spell out a message, each flag representing a letter.
• Individual flags have specific and standard meanings;^[3] for example, diving support vessels raise the "A" flag indicating their inability to move from their current location because they have a diver underwater and to warn other vessels to keep clear to avoid endangering the diver(s) with their propellers.
• 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.

During the Allied occupations of Axis countries after World War II, use and display of those nations' national flags was banned.^{[citation needed]} In order to comply with the international legal requirement that a ship identify its registry by displaying the appropriate national ensign, swallow-tailed versions of the C, D, and E signal flags were designated as, respectively, provisional German, Okinawan, and Japanese civil ensigns. Being swallowtails, they are commonly referred to as the "C-pennant" (German: C-Doppelstander), "D-pennant", and "E-pennant".^{[citation needed]}

The signal flags for M, T, V and X are respectively similar to the flags of Scotland, France, the Saint Patrick's saltire and the flag of Finland.

Letter flags and ICS meanings

Letter / radio name	Flag	Blazon	ICS meaning as single flag	Meaning when used with numeric complements
A Alfa		Swallowtailed, per pale argent and azure	"I have a diver down; keep well clear at slow speed."	Azimuth or bearing
B Bravo		Swallowtailed, gules	"I am taking in or discharging or carrying dangerous goods." (Originally used by the Royal Navy specifically for military explosives.)
C Charlie		Azure, a fess gules fimbriated argent	"Affirmative."[a][b]	Magnetic bearing
D Delta		Or, a Spanish fess azure	"Keep clear of me; I am maneuvering with difficulty."[b]	Date
E Echo		Per fess azure and gules	"I am altering my course to starboard."[b]
F Foxtrot		Argent, a lozenge throughout gules	"I am disabled; communicate with me."[c]
G Golf		Paly of six or and azure	"I require a pilot." By fishing vessels near fishing grounds: "I am hauling nets."	Longitude (The first 2 or 3 digits denote degrees; the last 2 denote minutes.)
H Hotel		Per pale argent and gules	"I have a pilot on board."[b]
I India		Or, a pellet	"I am altering my course to port."[b]
J Juliett		Azure, a fess argent	"I am on fire and have dangerous cargo on board: keep well clear of me." or "I am leaking dangerous cargo."
K Kilo		Per pale or and azure	"I wish to communicate with you."	"I wish to communicate with you by...": 1) Morse signaling by hand-flags or arms; 2) Loud hailer (megaphone); 3) Morse signaling lamp; 4) Sound signals.
L Lima		Quarterly or and sable	"Stop immediately."	Latitude (the first 2 digits denote degrees; the last 2 denote minutes.)
M Mike		Azure, a saltire argent	"My vessel is stopped and making no way through the water."[b]
N November		Chequy of sixteen azure and argent	"Negative."[a]
O Oscar		Per bend gules and or	"Man overboard."[b] (often attached to the man overboard pole on boats). With a sinister hoist, the semaphore flag.
P Papa		Azure, an inescutcheon argent	The blue Peter. In harbour: 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 Quebec		Or	"My vessel is 'healthy' and I request free pratique."
R Romeo		Gules, a cross or	No ICS meaning as single flag. Prior to 1969: "The way is off my ship; you may feel your way past me."	Distance (range) in nautical miles.
S Sierra		Argent, an inescutcheon azure	"I am operating astern propulsion."[b]	Speed (velocity) in knots
T Tango		Tierced in pale gules, argent and azure	"Keep clear of me."[b] Fishing boats: "Keep clear of me; I am engaged in pair trawling."	Local time. (The first 2 digits denote hours; the last 2 denote minutes.)
U Uniform		Quarterly gules and argent	"You are running into danger."[d]
V Victor		Argent, a saltire gules	"I require assistance."	Velocity in kilometres per hour.
W Whiskey		Azure, an inescutcheon gules fimbriated argent	"I require medical assistance."
X Xray		Argent, a cross azure	"Stop carrying out your intentions and watch for my signals."
Y Yankee		Bendy sinister of ten or and gules	"I am dragging my anchor."
Z Zulu		Per saltire or, sable, gules and azure	"I require a tug." By fishing vessels near fishing grounds: "I am shooting nets."[e]	Time (UTC). (The first 2 digits denote hours; the last 2 denote minutes.)

Notes

NATO number flags

Number	NATO flag	ICS flag	Blazons
0 Zero			NATO: Argent, five crosses hummetty azure in saltire ICS: Or, a pale gules
1 One			NATO: Gules, a fess or ICS: Argent, a torteau
2 Two			NATO: Or, a fess gules ICS: Azure, a plate
3 Three			NATO: Azure, a fess gules ICS: Tierced in pale gules, argent and azure
4 Four			NATO: Gules, a saltire argent ICS: Gules, a cross of Saint Philip argent
5 Five			NATO: Or, a saltire azure ICS: Per pale or and azure
6 Six			NATO: Bendy sinister of seven argent and azure ICS: Per fess sable and argent
7 Seven			NATO: Gules, a pale argent ICS: Per fess or and gules
8 Eight			NATO: Or, a pale azure ICS: Argent, a cross of Saint Philip gules
9 Nine			NATO: Azure, a pale argent ICS: Quarterly argent, sable, gules and or

Substitute or repeater flags allow messages with duplicate characters to be signaled without the need for multiple sets of flags.

The four NATO substitute flags are as follows:

First substitute	Second substitute	Third substitute	Fourth substitute

The International Code of Signals includes only the first three of these substitute flags. To illustrate their use, here are some messages and the way they would be encoded:

"N"
"O"
"NO"
"NON"
"NOO"
"NOON"
"NONO"
"NONON"
"NONNN"

Wikimedia Commons has media related to International Code of Signals.

• "How Ships Talk With Flags", October 1944, Popular Science
• John Savard's flag page. Collection of different flag systems.
• Freeware to aid memorizing the flags
• 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.