Semaphore

A visual signalling method that represents information through prescribed positions of movable arms, flags, lights or mechanical indicators.

In maritime flag semaphore, a person holds two flags at different angles to form letters, numerals and procedural signals. Other semaphore systems include mechanical railway signals and historical shore stations, so the general term is broader than the two-flag alphabet.

Visual semaphore depends on line of sight, correct orientation and a shared code. Range, darkness, weather, background and human error limit its reliability.