Method of visual signaling, usually with flags or lights.
Before radio, a semaphore system was widely used to send messages between ships. A person would stand with arms extended, moving two flags to specific angles to indicate letters or numbers. Before the invention of the telegraph, semaphore signaling with lights on high towers was used to transmit messages between distant points; messages were read by telescope. Modern semaphores have included movable arms or rows of light simulating arms, displayed from towers and used to signal railroad trains.