International maritime signal flags
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:
- each flag spells an alphabetic message, letter by letter.
- 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.
- 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 were banned. 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" (C-Doppelstander), "D-Pennant", and "E-Pennant".
Letter flags (with ICS meaning) 
"I have a diver down; keep well clear at slow speed."
With three numerals, azimuth or bearing.
"I am taking in, or discharging, or carrying dangerous goods." (Originally used by the Royal Navy specifically for military explosives.)
"I require a pilot."
When made by fishing vessels operating in proximity of 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.)
In harbour: "The ship is quarantined."
At sea: "You should stop your vessel instantly."
With four numerals, latitude. (The first two denote degrees and the rest minutes.)
"My vessel is stopped and making no way through the water."[b]
"My vessel is 'healthy' and I request free pratique."
"I am operating astern propulsion." [b]
With one or more numerals, speed in knots.
"Keep clear of me; I am engaged in pair trawling."
With four numerals, local time. (The first two denote hours and the rest minutes.)
- N and C together (No and Yes) is used as a distress signal.
- Also signallable on a ship's whistle using Morse code. See International Code of Signals.
- The Z flag was also famously hoisted by Adm. Heihachiro Togo at the 1905 Battle of Tsushima as the Japanese fleet prepared to engage the Russian fleet. In Japanese coding at the time, the flag meant, "The Empire's fate depends on the result of this battle, let every man do his utmost duty."
Additional meanings 
|Flag||NATO Nations Naval Meaning - Replenishment at Sea|
|Romeo||Romeo at Dip: Romeo flag is located 3⁄4 way up toward the point of the hoist. On the control ship, it means, "I am steady on course and speed and am prepared to receive you alongside on side indicated." On the approaching ship, it means, "I am ready to come alongside."
Romeo Close Up: Romeo flag is at the top of the hoist, touching the point of the hoist or as high as it will go. On the control ship, it means, "I am ready for your approach.: On the approach ship, it means, "I am commencing my approach."
Romeo Hauled Down: This means the first messenger is in hand for controlling and receiving ship.
|Bravo||Bravo at Dip: Bravo flag is located 3⁄4 way up toward the point of the hoist. On the delivery ship, it means, "I have temporarily stopped supplying." On the receiving ship, it means, "I have temporarily stopped receiving."
Bravo Close Up: Bravo flag is at the top of the hoist, touching the point of the hoist or as high as it will go. On both ships, it means fuel or explosives are being transferred.
Bravo Hauled Down: On both ships, it means delivery is complete.
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:
See also 
- Character encoding
- Day shapes
- Diver down flag
- England expects that every man will do his duty
- (Japanese) The fate of the Empire depends upon today's battle: let every man do his utmost
- Flag of Germany after World War II (C-Pennant)
- Flag semaphore
- International Code of Signals
- List of international common standards
- NATO phonetic alphabet
- Signal lamp
- International Marine Signal Flags
- AB Nordbok. "The Lore of Ships", page 138. New York: Crescent Books, 1975.
|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.