Eight places an arm can be
Each arm points in one of eight directions, forty-five degrees apart, like a clock face. Two arms, eight positions each, and that is enough for the alphabet with room left over for numbers.
Six letters need one arm brought across the body, so both flags end up on the same side. The flags never overlap except at rest, when both arms hang straight down.
Red and yellow at sea, blue and white on land. The colours do not carry meaning; they are there to make the arm easier to see.
The whole alphabet
It is built in circles rather than in alphabetical order. A to G use one arm with the other hanging down. Then the right arm parks at the A position and the left arm sweeps round for H to N. Then it parks one place further on, and round again.
J is the odd one out, which is why it is skipped in the second circle. It doubles as the sign that says the letters are letters, not numbers.
Where it came from and where it still is
Before hand flags there were towers: a pole with two great moving arms, which is why the positions are circular. Chains of those towers carried messages across countries faster than a horse could run, and semaphore signals were used at the Battle of Trafalgar.
It is still in use at sea today, when two ships pass fuel and stores between them and nobody wants to broadcast a radio signal. Two arms, no batteries, nothing to intercept but line of sight.
And that is the catch. Everyone in sight reads it. Semaphore carries a message beautifully and cannot keep one, which is exactly why the ciphers exist.
Back to signals Learn morse The full flag chartRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.