International Code of Signals

International Code of Signals, originally a specific method of communication between ships, or between ship and shore, by hoisting flags and pennants, which in a group, or singly, conveyed a message. However, the phrase later included other means of communication at sea, such as Morse code.

The original code was founded in 1817 on the one invented by Captain F. Marryat, and at first comprised only fifteen flags and pennants. By 1855 it was being widely challenged by codes developed in France by Captain Reynold-Chauvaney, in Great Britain by Rohde (1836) and Watson (1842), and in the USA by Rogers, and an international committee was set up in 1856 to try to reach agreement on a single code for universal use. The committee's final recommendation was based almost entirely on Marryat's original flags and it published a Commercial Code of Signals which received universal recognition. In 1887 the existing code ran into difficulties because the adoption of signal letters for ship identification, and additional coloured flags and pennants, had to be incorporated into the code. This was universally agreed in 1900, and brought into use in 1902. The current code, which came into effect in 1969, provides for nine languages—English, French, German, Greek, Italian, Japanese, Norwegian, Russian, and Spanish.

The system provides for one flag for each letter of the alphabet and eleven pendants, numerals 0 and 9, and the answering pendant; and three triangular-shaped flags, the First, Second, and Third substitutes, which are used to repeat one or more letters in a group. Each letter flag has a meaning which, when hoisted by itself, indicates a very urgent message, or one which is important or common. For example, the letter ‘O’ indicates there is a man overboard, the letter ‘B’ that the ship flying it is taking in, or discharging, or carrying dangerous goods.

These single signals were well recognized by professional seamen, but when it came to decoding groups of two, three, and four flags (for ship identification) a code book was used. Each signal was normally a complete message in itself, but sometimes what was known as complements were used to alter a message, or to make it more specific. For example, the two-letter group KT meant ‘you should send me a towing hawser’ but when the numeral 1 was added it meant ‘I am sending towing hawser’; and the letter group CB meant ‘I require immediate assistance’ but when the numeral 4 was added it meant ‘I require immediate assistance; I am aground’. There were many other complements as well as instructions for signalling such information as dates and times and latitude and longitude so that it was clear and unambiguous.

In today's world of instant satellite communications, signalling by flag is almost a thing of the past, though some of the single flags, such as the blue peter, are still used. However, unlike semaphore, the International Code of Signals has not yet been made redundant by the International Maritime Organization which oversees international maritime communications.

See also signals at sea; tackline.