Flag signaling

Flag signaling is visual communication by hoisting the flags and pennants of the International Code of Signals, where each hoist carries a meaning fixed by the Code so ships of different languages can pass a message. The Code uses 26 alphabet flags, 10 numeral pennants, 3 substitutes, and the answering pennant. A single flag carries an urgent or common signal, for example flag O for man overboard, flag A for diver down keep clear; two-flag and three-flag groups give a wider vocabulary. The reader looks up the hoist in the International Code of Signals book. Flag signaling complements semaphore, flashing-light Morse, and radio, and remains the back-up when radio fails.