Maritime flag signalling

In the early days of sail, the use of signals to communicate between ships was primitive, as seen by one admiral's instructions to his fleet in 1530: Whensoever, and at all tymes the Admyrall doth shote of a pece of Ordnance, and set up his Banner of Council on Starrborde bottocke of his Shippe, everie shipps capten shall with spede go aborde the Admyrall to know his will.

[1]By 1653, the Royal Navy had issued instructions by which an admiral could signal various orders by hoisting flags in various locations on his ship.

A ship's message had to be approved by the officer of the watch, and his system was augmented and changed in various ways over the following century.

Bourdonnais proposed hoisting the flags in groups of three, making a thousand possible messages that could be transmitted by reference to a code book.

The Chappes developed an optical land telegraph which used a numerical code book with many thousands of messages.

De Bigot's book was published in England in 1767, but it was several decades before the Royal Navy developed their own system.

[10][12] Popham's code was famously used for the "England expects that every man will do his duty" signal at Trafalgar by Nelson: for this, a team of four to six men would have prepared and hoisted the flags onboard Lord Nelson's flagship HMS Victory, the whole process taking about four minutes.

[16] During World War I, there was an unprecedented need for ships to communicate, merchant as well as naval, but the ICS was found wanting: "It was not international.

[18] Additional changes in 1969 greatly reduced the Code (dropping the Geographical and Vocabulary sections), and more narrowly focused it on communications related to safety of navigation.