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
Z flag Z flag The Z flag is one of the international maritime signal flags. In the system of international maritime signal flags, part of the International Code of Signals, the Z flag stands for the letter Z ("Zulu" in the NATO Alphabet) when used in letter-by-letter alphabetic communication. When used alone, it means "I require a tug" or, when used by fishing vessels near fishing grounds, "I am shooting nets". The Z flag when combined with four number flags (The leading two denoting hours, the trailing two denoting minutes) indicates Z Time (also called Zulu Time), a military and maritime term