Telegraphy the sense of a system that can transmit arbitrary messages over arbitrary distances. Lines of signalling relay stations can send messages to any required distance, but all these systems are limited to one extent or another in the range of messages that they can send. A system like flag semaphore, with an alphabetic code, can certainly send any given message, but the system is designed for short-range communication between two persons. An engine order telegraph, used to send instructions from the bridge of a ship to the engine room, fails to meet both criteria; it has a limited distance and
International maritime signal flags with duplicate characters to be signaled without the need for multiple sets of flags. The four NATO substitute flags are as follows: The International Code of Signals includes only the first three of these substitute flags. To illustrate their use, here are some messages and the way they would be encoded: International maritime signal flags International maritime signal flags refers to various flags used to communicate with ships. The principal system of flags and associated codes is the International Code of Signals. Various navies have flag systems with additional flags and codes, and other flags are used in special uses,