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Code flags, semaphore, Morse and the flags flown at a beach. The International Code of Signals is unusual in this catalogue for being adopted by treaty, which means a flag's meaning can be stated flatly — with one crucial qualification that every record repeats: a flag flown alone carries a defined message, and the same flag inside a two- or three-letter group is only a letter. Semaphore and Morse are transliterations with no vocabulary of their own; treating their characters as meaningful is the same error as reading a card suit as a social class. The beach flags are the ones most readers will actually meet, and the pair worth knowing inverts what the colours suggest: red over yellow is where to swim, plain red is do not enter.
The treaty-adopted flag code every seagoing vessel carries.
White and blue swallow-tail: a diver is down, keep clear at slow speed.
Plain red swallow-tail: dangerous cargo.
Yellow with a broad blue band: keep clear, I am manoeuvring with difficulty.
Red and yellow diagonal: man overboard.
White with a red saltire: I require assistance.
Blue, white and red rectangles nested: I require medical assistance.
Plain yellow: my vessel is healthy and I request free pratique.
Triangular pennants that repeat a flag already flying.
The full string of code flags flown for ceremony, spelling nothing.
Two handheld flags whose angles spell letters, one at a time.
Dots and dashes — and a timing relationship that the printed form hides.
Red-and-yellow, red, black-and-white chequered — and the one that is not a warning.