International Code of Signals (INTERCO)

The International Code of Signals (INTERCO) is the IMO-published code that lets ships exchange safety-of-navigation and emergency messages across language barriers using standard signal groups. A message can be sent by flag hoist, flashing-light Morse, sound Morse, semaphore, voice over radiotelephone, or signaling lamp, the meaning staying fixed by the Code. It is built around single-letter signals for the most urgent meanings, two-letter signals for general messages, and three-letter groups beginning with M for medical advice. First adopted in 1965 in its present form, it remains carriage-relevant for SOLAS ships and is the fallback when plain-language radio is not workable. Crews drill the single-letter signals because those carry the time-critical meanings.