How the International Code of Signals works
Unlike Morse or flag semaphore, marine signal flags don't encode letters through position or rhythm - each letter simply is a flag, with its own fixed colours and pattern. A ship can spell out a word by hoisting one flag after another, or fly a single flag (or short combination) to send a whole standard message, like "O" for man overboard.
Where did signal flags come from?
The modern International Code of Signals dates to the 19th century, when merchant fleets needed a shared visual language that worked across nationalities without a common spoken language. Five colours - red, yellow, blue, black and white - combine into 26 letter flags and 10 numeral pennants, still carried on ships today alongside radio.
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