UKHO

The International Code of Signals

 

INTERCO is a maritime system of codes and signals that vessels use to communicate messages to each other regarding safety of navigation.

Signals can be sent by hoisting flags, signal lamps, flag semaphore, radiotelephony or radiotelegraphy.

 

Prior to 1857 the most widely used flag codes was call the Marryat Code.  The modern International Code of Signals was first drafted by the British Board of Trade in 1855 and published in 1857 under the name of the Commercial Code.

Eighteen signal flags were used to make more than 70 000 different messages. Vowels were omitted so that words that could possibly be objectionable in certain languages would be avoided.

 

The code was revised by the Board of Trade in 1887 and modified again in 1889. The new International Code of Signals came into worldwide use in 1901. The number of flags had increased to 26 plus a code pennant. The new flags represented the vowels and the letters XYZ that had previously been omitted.

 

What is it used for?

The purpose of the INTERCO is to give mariners a way to communicate in situations related to safety of navigation and people in a manner that overcomes the difficulty of spoken language. This has been done by establishing a standardized alphabet (letter A – Z and ten digits) together with a spoken form of each letter. This alphabet was then associated with a set of standardized flags.  Different combinations of these alphanumeric characters have been assigned as codes for various messages. For example, if the vessel’s radio is not working and the master wishes to communicate with another vessel, he can raise the Kilo flag (see diagram) or send a Morse Code equivalent by flashing light – this has been assigned the standard message for “I wish to communicate with you”.

 

Groups of signals

The signals are divided into three groups:

Single letter signals – these are important, urgent and common

Two letter signals – these are sometimes followed with a numeric to supplement of modify the message.

Three letter signals – these begin with “M” and are the medical signal codes.

 

Sometimes additional characters are added to indicate things like bearing, distance, course, quantities, latitude, longitude, date and time.  Provision is also made for spelling words and for indicating use of other codes.

 

In order to avoid having to hoist multiple sets of signal flags, the code uses three “repeater” flags. These flags repeat the flag at the indicated position – for example, to signal MAA (“I request urgent medical advice”) the Mike, Alfa and second repeater flags would be flown. The substitute flag shows a repeat of the second character. See below:

Numerals

0 Nadazero [ˌna.daˈzɪə.roʊ]		This and following used as numbers to complement other signals.
1 Unaone [ˌuː.nəˈwʌn]
2 Bissotwo [ˌbɪs.oʊˈtuː]
3  Terrathree [ˌtɛr.əˈtri]
4 Kartefour [ˌkɑː.təˈfɔː.wə]
5  Pantafive [ˌpan.təˈfaɪf]
6 Soxisix [ˌsɔk.siˈsɪks]
7 Setteseven [ˌsɛ.təˈsɛv.ən]
8 Oktoeight [ˌɔk.toʊˈeɪt]
9  Novenine [ˌnoʊ.veˈnaɪ.nə]

Various

1st Substitute		Used to repeat the first flag of a hoist later within the same hoist. This and the other two substitutes allows any four letter or number combination to be sent using only one set of flags.
2nd Substitute		Used to repeat the second flag of a hoist later within the same hoist.
3rd Substitute		Used to repeat the third flag of a hoist later within the same hoist.
Code/Answer Pennant		At the dip (about half-way up the halyard): Ready to receive message Close up: Message has been received and understood (the flag is then hauled back at the dip to receive the next hoist) Hauled down: Signals end of message. With numerals: Decimal point By a warship: When flown over a hoist, indicates the message is to be read according to the ICS.

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