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International Code of Signals (Overview)

Last modified: 2020-04-19 by rob raeside
Keywords: international code of signals | signal flags | maritime signal flags |
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International Code of Signals (Overview)

The International Code of Signals was first drafted in 1855 by the British Board of Trade and subsequently published in 1857 as a means of maritime communications. The original publication showed 17,000 signals using 18 flags, part of which was specific to the United Kingdom and another part that contained universal signals to be used by all nations. Adopted by most sea-faring nations, the system was revised in 1932 to include seven languages: English, French, German, Italian, Japanese, Spanish, and Norwegian.

The Fourth Assembly of the Intergovernmental Maritime Consultative Organization revised the code in 1965 which became effective on January 1, 1969. This revision added Russian and Greek to the languages already included and adopted a new radiotelephone code. Each signal has a complete meaning.
Jorge Candeias, 31 August 1999

The site of the Mystic Seaport Library has put on line the following brochure: Report of the Committee Appointed by the Lords of the Committe of Privy Council for Trade to Inquire into and Report upon the subject of a Code of Signals to be used at Sea published in London by HMSO, 1857.

Fourteen pages in all, this makes interesting reading and provides background information on the machinery leading to the adoption of signal flags.

Thirteen official and more less official codes were debated as were the various qualities a system should have. A 'Numeral' code's disadvantages are listed and some simple mathematics displayed e.g. , the number of permutations. Maryatt's code flags are recommended albeit with some variations.

The report (accompanied by a separate signal book) did not only address British usage but foreign as well, in fact, it invited contributions from other nations along the lines specified. Lastly, it was decided not to burden the system with a list of ship's names.

A few appendices further develop some of the above.
Jan Mertens, 9 August 2005

All signal code flags are square, although A and B are broadly swallowtailed. The colours are as follows:

  • A = vertically divided, white on hoist, blue on fly.
  • B = red
  • C = horizontally divided, blue-white-red-white-blue, ratio 1:1:1:1:1
  • D = horizontally divided, yellow-blue-yellow, ratio 1:2:1
  • E = horizontally divided, blue over red
  • F = white field, with red square rotated 45 o on to its corner, extending to the edges of the flag
  • G = vertically divided (from hoist) yellow-blue-yellow-blue-yellow-blue
  • H = vertically divided white on hoist and red on fly.
  • I = yellow field, small black circle in centre
  • J = horizontally equally divided blue-white-blue
  • K = vertically divided, yellow on hoist, blue on fly
  • L = quartered, first and fourth yellow, second and third black.
  • M = blue field, white saltire-style cross
  • N = chequered, four rows and four columns, alternating blue and white, beginning with blue in the upper hoist
  • O = diagonally divided, yellow in the lower hoist, red in the upper fly.
  • P = blue field, small white square centred on it
  • Q = yellow; the quarantine flag
  • R = red field, yellow cross
  • S = white field, small blue square centred on it
  • T = vertically equally divided, red-white-blue from hoist to fly
  • U = quartered, first and fourth red, second and third white
  • V = white field, red saltire-style cross
  • W = blue field, white square centred on it, small red square centred in the white
  • X = white field, blue cross
  • Y = diagonally striped, stripes rising from lower hoist to upper fly, yellow-red-yellow-red-yellow-red-yellow-red-yellow-red
  • Z = diagonally quartered, yellow in upper sector, black along hoist, red in lower sector, blue in fly sector.

Signal Flags in Combinations

Ship signal flags can also be grouped in pairs to make new combination meanings. Here is a list I took from Dictionaire Complet et Illustré Petit Larousse, 1996

  • NC      I'm in danger
  • ED      Your help signal was understood
  • KP      Tow me to port
  • FO      I'm standing by you
  • PM      Follow me
  • IR      Stay at large
  • JG      I'm astray
  • NA      Navigation prohibited
  • KN      I can't tow you

Aside from that, the flag for "shipwreck" is plain black. Somehow I fail to imagine what is the use of hoisting a flag on a drowning ship, but... :-\
Antonio Martins, 1998 February 03

The international signal code was elaborated by OMCI in 1965. They presented signals of 1 letter (emergency or frequent use), 2 letters, 3 letters (medical part), with possibly 1, 2, or 3 numeral completive. You can find them in all major hydrographic offices. in the book I have, proportions showed are (approx): 4 / 3 for letter flags, 16 / 3 for numeral pennants, 5 / 4 for substitutes. The black flag does not appear in it any more.
Armand Noel du Payrat, 1998 February 14


I asked a Flagmakers firm ("Industrial Velera Marsal S.A.") and they make 3 sizes measuring: 1.98x2.41 m, 1.37x1.68 m and 0.76x0.91 m, which they assure are "official". I didn´t find any proportions kept at the 3 sizes. They are close to 8-10, but a little more "squared" than that.

Maritime letter flags, as far as I know, go back to Sir Home Popham, who published "Telegraphic Signals or Marine Vocabulary" in 1800, with a larger version in 1803, and another expanded edition in 1812.

This was used by Nelson to signal his fleet before the beginning of the battle of Trafalgar, the 21 of October, 1805, the famous message: "ENGLAND EXPECTS THAT EVERY MAN WILL DO HIS DUTY". Certainly those flags were 1-1 in proportions and can be seen in several books.

After many manuals and codes, the actual international signal flags developed from Captain Frederick Marryat´s "Code of Signals for the Merchant Service".

This actual international code is what we need to find out if it has construction sheets.

In Whitney Smith´s 1975 book, page 86, letter flags are drawn in prop. 8-10.
Jose C. Alegria, 25 August 1999

I am not a specialist of signal flags (Album des Pavillons presents all national flags, ensigns and markings at sea, but not signal flags) yet I had a look at the technical specifications of French Navy and British Royal Navy : signal flags have the following measures (in cm).
French Navy 198 : 244
137 : 168
76 : 91
British Navy 183 : 229
114 : 152
102 : 122
61 : 76
46 : 53
30 : 38
Armand Noel du Payrat, 25 August 1999
Here are the correct proportions for the International Ship Code Flags:
  • Letters A & B-1:1.5 (swallowtailed)
  • Letters C - Z (& the US Navy's set of 10 numerical square flags)-1:1 (square)
  • The 10 ICS numerical-& the code/answering-pennants-5:9 (tapered at fly)
  • The 4 substitutes (repeaters), the books-on-board (white with a blue reader), & the PROMPT (yellow/green/yellow-new!!) pennants-7:11 (triangular).
Robert Lloyd Wheelock, 25 August 1999

In Flags at Sea, Timothy Wilson wrote; "The most common sizes for signal flags of the International Code nowadays (1986) are: 78 inches by 96 inches, 54 inches by 66 inches, and 30 inches by 36 inches."

The sizes offered in a current catalogue are (all in inches): 9x12, 12x18, 18x21, 24x30, 30x36, 43x54, 48x72.

Marryat suggested that his flags should be 6 feet by 8 feet, with pennants 4 feet by 18 feet.
David Prothero, 28 August 1999

The US Navy page on signal flags, also shows what look like 1:1 proportions. Although the depiction of the "Romeo" flag looks to me like the cross is too narrow, so use your best judgment on reliability of the other representations. The page also shows United States Navy vs. international meanings of individual flags, for those interested.
Joseph McMillan, 31 August 1999

I visited the Bornholm Museum in Rønne, on Bornholm, during my vacation, and they had this overview of the signaling flags in different version of the code. Signaling flags being an interest of mine, I just had to jot them down:

If I'm deciphering my all-too-small notes correctly, the 1867 version had:

B   Like the current flag
C   White with a red dot (as the current "1", but shaped more or less like the current repeaters)
D   Blue with a white dot (as the current "2", but the same shape as its "C")
F   Red with a white dot (its "C" in reverse)
G   Yellow before blue  (as the current "K", but shaped like its "C", and with the 
    yellow only approx. 2/5 of its length)
H   Like the current flag
J   Like the current flag
K   Like the current flag
L   Quartered blue and yellow
M   Like the current flag
N   Like the current flag
P   Like the current flag 
Q   Like the current flag
R   Like the current flag
S   Like the current flag
T   Like the current flag
V   Like the current flag
W   Like the current flag

Answering pennant Like the current flag (However, I'm not sure about this one: Other sources, displaying the 1901 code picture a sharp-tipped flag, like the "C" in this code.

I knew there were no vowels in this code, but it turns out there were no X or Z either. Well, considering that the code started out as English [It did, didn't it?], and these letters are basically non-native for English, this might make sense.

OK, returning to my notes, and the di Pietri system, these were the changes to the flags in 1901:

A   Like the current flag
E   Columns of red, white and blue (like the "T", but shaped like its "C") 
F   Red with a white cross (shaped like its "C")
I   Like the current flag
L   Like the current flag
O   Like the current flag
U   Like the current flag
X   Like the current flag
Y   Like the current flag
Z   Like the current flag

Adding the missing letters. The "F" is changed to avoid confusion with the other dark-coloured pennant with white dot, I expect. Why did they change the "L"? Maybe a partly folding "L" would look too much like a "K"?

In 1933 all changed to their current flags. The pennant shaped letters were replaced. The digit pennants, with their obtuse tips, were introduced, as well as the repeaters. [How did they repeat before this, then? Or was the code build to avoid repetition?] From now on the answering pennant seems to have its shape like a slightly longer version of a digit pennant.

The chart didn't mention any other signals. If the black wreck flag was ever part of the code, it wasn't shown here, nor did it show when any of the other signal flags were introduced.
Peter Hans van den Muijzenberg, 10 August 2003

Comparing the 1913 International Signals Code depicted therein with the current ICS, I noted that the flags for the letters C, D, E, F and G are pennants. At some point the rectangular flags with the quite different designs that they are in the current International Code of Signals. All the other alphabetical flags of the 1913 code are still the same in the current ICS.

Incidentally, these five pennants with their designs survived in the modern ICS as the numeral pennants 1,2,3,4 and 5.
Andre Burgers, 8 September 2004

The change came into force on 1st January 1934. In the previous 1901 Code the alphabet flags were used to represent numbers, from 2 to 27. 1934 was also when the three substitute flags were introduced.
David Prothero, 9 September 2004

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