The catcher's signs
A catcher flashes four signs. Three are meaningless. One says what the pitcher should throw, and which one it is was agreed before the game: today it might be the second sign, tomorrow the third.
Everyone in the stadium can see the signs. Cameras record them. It does not matter, because the code is not the signs. The code is the rule about which sign to read — and that rule lives in two heads.
That is the same shape as a null cipher: most of what you can see is there to waste your time.
Null ciphersThe runner on second base
There is one place on the field where a fielder can see the catcher's hands from behind: standing on second base. A runner there can watch the signs, work out which one counts, and tap his helmet to tell the batter what is coming.
So teams change the rule mid-game, or add a sign that means the rule has changed. Watch a code being attacked and defended in real time, in front of a crowd, at a baseball game.
In 2022 the major leagues gave pitchers a radio in the cap so the catcher could just say it. Two hundred years of signalling, retired by a microphone.
Hands that are a language
Sign languages are not codes for English. They are full languages with their own grammar, built by deaf communities, and they have alphabets for spelling names and places. Those alphabets are worth learning properly rather than from a decorative chart, and the best place to learn one is from people who use it.
Divers have their own signals because you cannot speak underwater. Referees have theirs because you cannot shout over a crowd. Airport marshals have theirs because you cannot be heard over a jet. All the same instinct: when the voice cannot get through, the hands will.
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