Flags have been used at sea for centuries. They allow ships to identify the country another ship hails from at a glance. On warships, flags were used to indicate that an admiral was on board or that other ships should follow it into a fight. Later, a system of sending signals from ship to ship using flags arose. Since the nineteenth century, ship-to-ship flag signals are set out in the International Code of Signals.

Visual signs for long distance

The first Dutch code letter dates from 1558. It describes the flags and pennants used on the fleet of vice-admiral Adolf of Burgundy. Sea Beggars also used flag signs. At that time, the use of flags was very limited, and there was no system governing their use. Ships were slow, and most orders were issued before the ship sailed, during war councils on board the commanding flagship.

Michiel de Ruyter

Over the years, the size of fleets grew, and a need emerged for a more detailed system of signals. In 1639, Maarten Harpertszoon Tromp used red, white and blue signal flags. Michiel Adriaanszoon de Ruyter used 44 flag signals, a great deal more than any of his predecessors. The signals that would be used would be arranged in advance of the voyage, and then copied down precisely into code books or instructions. Squadrons were then very well prepared for a sea battle. The disadvantage of the signal flags was that they could no longer be flown if the mast was shot off or broken.

International code for signals

It was only in 1781 that a system of signalling with flags was introduced that allowed messages to be sent back and forth. In fact, the signal codes used by the navy involved a simple system of numbers. The signal given represented a number that corresponded to a word in the code book. Whole sentences to be transmitted using this word code. An extension of this system was documented in 1820 in the Telegraphic Signal Book. The development of various types of signals given with flags continued into the First World War (1914), and was standardised thereafter. The special navy flags were replaced by the flags from the International Code of Signals. This book is published in nine languages, and contains instructions for signalling. Any ship larger than 150 gross tonnes is required to have this book on board.

Sailingraces today still use the same flags

The flags from the International Code of Signals are still used in the navy and merchant marine. Even in this day and age of computers, and mobile phones, the use of flag signals remains important. For example, if a ship is flying the Blue Peter (flag signal P), everyone knows it is leaving port. Even today, the same flags are used for sailingraces. The flags show the fleet when they are allowed to cross the startingline or when the course is changed.