From at least the time of the Athenians, sailors have hoisted flags to communicate to other ships in the fleet. By introducing the use of flags as signaling devices, ancient sailors were able to effectively communicate over greater distances and during adverse conditions.
As the use of nautical flags expanded, their value grew as a method of conveying one’s identity to unknown vessels from afar, providing wary captains a vital tool for recognizing friend or foe at a safe distance. During times of conflict, signal flags proved an invaluable advantage in coordinating an armada’s movements against enemies with inferior communication abilities.
The development of nautical signal flags had a dramatic impact on the history of naval operations. As the sophistication of signal flag systems grew, ever more complex fleet maneuvers were made possible. However, with increased sophistication and flag vocabulary, the opportunity for error also increased. An example of this can be seen in the experience of the British Admiralty in 1800 when it adopted an expanded signaling system invented by Rear Admiral Sir Home Popham. The new system added for the first time alphabetic flags to the then-current numeric flag system and created a dictionary of some 3,000 defined words and sentences. The Popham system triggered an explosion in maritime vocabulary and by 1813 had grown to contain over 6,000 defined phrases and 60,000 words. It was the Popham system by which Lord Nelson sent his immortal message at the 1805 Battle of Trafalgar: “England expects that every man will do his D-U-T-Y.”
Over time, various competing signaling systems began to create issues on the high seas as flag combinations often had entirely different meanings depending upon the nationality of the ship. The risks of miscommunication were particularly dire when warships of competing navies approached one another. So, in 1870 the first major step was taken to create a single internationally recognized flagging system with the establishment of the International Code of Signals or ICS. The system ultimately became ubiquitous and in 1965 the United Nations’ International Maritime Organization assumed responsibility for its administration. The system today uses 26 alphabetical flags, 10 numeral pendants, one answering pendant and three repeaters in one-flag to six-flag message combinations to convey many thousands of defined messages. Most alphabetic flags also have their own individual meaning such as (F) Disabled, (I) Altering Course, or (V) Require Assistance.