The illustration below illustrates the flags used for the International Code of Signals, and additional flags currently in use by the U.S. Navy and apparently by the British Navy as well:
The flags of the International Code of Signals, although they represent the alphabet, aren't normally used for English-language text. Instead, they are used either to give the call letters of a ship, which are allocated internationally by subdividing the alphabet in much the same way as is used for radio stations, and which are four or five letters long, or for indicating codewords in the International Code of Signals. Codes of one or two letters in length indicate emergency situations, three-letter codewords serve general purposes. Because of the increased ease of radio communications, it appears that the three-letter codes, except for medical codes starting with M, have been removed from the current version of the International Code of Signals. However, some of the two-letter codes may be followed by a digit, called the complement, which adds more specific information.
Some of the flags are named in the illustration by a two-letter abbreviation:
The Repeater/Substitute flags allow any combination of five letters to be hoisted with only a single set of flags: hence, 2R means a duplicate of whatever was signified by the second flag. There are only one of each substitute flag in a set, too; thus, these flags are daisy-chained when necessary. For example:
AXAXA A X 1R 2R 3R AXAXX A X 1R 2R 4R AXAAA A X 1R 3R 4R
Some sources only show the first three of these flags.
The extra naval flags are the following:
Some other sites which include the U. S. Navy numeric flags are listed below.
I had originally thought the square naval numerical flags were strictly for use in identifying fleets, however, this site which also shows these flags notes that they are the normal flags for the digits in the Navy, with the International Code flags for the numbers having the meanings "Pennant one" rather than "One", and so on. Other references have made it clearer: the Naval numerical flags, like the letter flags of the International Code, are usually used to indicate the numerical codes for phrases, in this case from the Navy Code rather than the International Code. The Flotilla and Subdivision flags, though, are not shown there. The Naval flags are also visible at this page and this site as well. As well, this site notes the fourth repeater as specifically naval. Finally, this site shows the other U. S. Navy flags, besides the numeral flags, that the diagram includes.
This illustration includes the flags for the signal flag system developed by Lord Richard Howe, and their slight modification for use with a longer code of signals later developed by Sir Home Popham which was first used in the famous Battle of Trafalgar involving Admiral Horatio Nelson. (Note that the flag for the numeral 4 was reversed between the two codes; this is not an error in the diagram.) Also shown are the flags of the Marryat code, which immediately preceded the current International Code of Signals.
The web site of the Peabody Essex Museum contains a page which illustrates several flag code systems, including some not shown here.
While the Peabody Essex Museum site has a number of older signal flag systems illustrated, it omitted the one used by Nelson. Although I could not find a table of that code on the web, this site gives the message hoisted by Nelson in its numerical form, and this site includes a picture of the hoist.
Apparently, the British Navy code of Nelson's time required only one substitute flag, as codes requiring a second repeater were omitted.
Other flags used with the old British Navy code are:
Other flags used with the Marryat code are:
The Howe code adds flags for some other purposes:
And finally, some flags used for other signalling purposes, but not used as part of a signal flag set (although similar in appearance) appear in the rightmost column of the illustration:
Copyright (c) 1998, 1999, 2000 John J. G. Savard