There's a lot of commentary here about Morse code being possible by tapping on a wall, which it is if and only if both sender and receiver are already trained i

There's a lot of commentary here about Morse code being possible by tapping on a wall, which it is if and only if both sender and receiver are already trained in Morse. Problem is, your average prisoner isn't, the encoding is not at all obvious (because it's designed for speed of use, not ease of learning), and they can't just look up it up on Wikipedia when in solitary confinement, This is why simpler tap codes exist: A = 1 tap, B = 2 taps etc is self-evident, and from there it's only a small leap to the Polybius square 5x5 tap code covered in Wikipedia.