Eight flags and what they say
There is a flag for every letter, so a ship can spell. But each flag also means something entirely on its own, and that is the clever part: one piece of cloth carries a sentence.
Why a codebook beats spelling
Spelling MAN OVERBOARD takes thirteen flags, thirteen hoists, and time you do not have. The O flag says it in one, and every ship in sight understands instantly, whatever language its crew speaks.
That is a codebook: an agreed list where one short thing stands for one long thing. Nothing is scrambled. It exists to save time, and it works precisely because everybody has the same book.
Two flags together mean something else again, and three something else, which is how a few dozen flags cover thousands of messages. The same idea as a nomenclator, which ran European diplomacy for centuries, and the same idea as an airport code or a hospital's crash call.
The book cipherOne flag with a life of its own
P is the blue peter: a blue flag with a white rectangle, flown when a ship is about to sail and everyone ashore should get back aboard. It has been flying for two hundred years, and the phrase followed it into ordinary English.
Q is plain yellow and asks to be let into port because there is no illness aboard. In the days of quarantine that flag was the difference between docking and waiting offshore for forty days, which is where the word quarantine comes from: forty.
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