systems of signals used to transmit information to and from ships. Marine signals can be sound signals, using bells, whistles, horns, sirens, and other devices, or visual signals. The latter include flag signals (both international and other codes), semaphores (flag semaphores and mechanical semaphores as well as signal panels used in aviation and other devices), signal figures (balls, cones, cylinders, and strips raised on halyards, stays, or acorns), and light signals (ship’s lights and searchlights).
There are international and local codes in the form of alphabets (semaphore or Morse) and conventional signs. Light signals are used for traffic control in navigable channels. Sound signals are used during the passing of ships. For example, one short blast means “altering course to starboard”; three short blasts means “engines going full astern.”