The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships. It is a component of the International Code of Signals (ICS).

 

There are various methods by which the flags can be used as signals:
1. Each flag spells an alphabetic message, letter by letter.
2. Individual 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.
3. One or more flags form a code word whose meaning can be looked up in a code book held by both parties.
4. 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".

 

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.

 

Letter flags with ICS meaning:

A (Alfa):    "I have a diver down; keep well clear at slow speed."
B (Bravo):    "I am taking in, or discharging, or carrying dangerous goods".
C (Charlie):    "Affirmative".
D (Delta):    "Keep clear of me; I am maneuvering with difficulty."
E (Echo):    "I am altering my course to starboard".
F (Foxtrot): "I am disabled; communicate with me."
G (Golf):    "I require a pilot."
H (Hotel):    "I have a pilot on board."
I (India):    "I am altering my course to port."
J (Juliet):    "I am on fire and have dangerous cargo on board: keep well clear of me."
K (Kilo):    "I wish to communicate with you."
L (Lima):    In harbour: "The ship is quarantined."
                At sea: "You should stop your vessel instantly."
M (Mike):    "My vessel is stopped and making no way through the water."
N (November):    "Negative."
O (Oscar):    "Man overboard."
P (Papa):    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):    "My vessel is 'healthy' and I request free pratique."
R (Romeo):    "The way is off my ship."
S (Sierra):    "I am operating astern propulsion."
T (Tango):    "Keep clear of me; I am engaged in pair trawling."
U (Uniform):    "You are running into danger."
V (Victor):    "I require assistance."
W (Whiskey):    "I require medical assistance."
X (Xray):    "Stop carrying out your intentions and watch for my signals."
Y (Yankee):    "I am dragging my anchor."
Z (Zulu):    "I require a tug."