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).[1]
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There are various methods by which the flags can be used as 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".
* ^ 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."
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 |
To illustrate their use, here are some messages and the way they would be encoded:
| "N" | |||||
| "O" | |||||
| "NO" | |||||
| "NON" | |||||
| "NOO" | |||||
| "NOON" | |||||
| "NONO" | |||||
| "NONON" | |||||
| "NONNN" |
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