Chart Showing the International Signal Flags
International signal flags have been used to communicate between merchant ships for almost two centuries, though the system of communication has changed. The first widely used code was created by Captain Fredrick Marryat, Royal Navy, in 1817, using numbers only. Numbers were assigned to ships and then later letters after the introduction of a new code in 1857. These were published in ship registers, with each ship having a unique set of letters, just as automobiles have unique license plates. In 1857, when the system was revised, some 40,000 were needed. The system created at that time, called the Commercial Code, is the basis of the revised International Code seen here. Besides identification, requiring code books, signal flags were used for a wide range of communications.