Tap Code (Prison) Cipher

The tap code is the Polybius square transmitted by sound. Each letter is a row and a column of the five-by-five grid, and each number becomes that many knocks with a pause in between. The letter C, first row and third column, is one knock, pause, three knocks. It became known through use among prisoners of war in Vietnam, who passed it along through cell walls.

The page converts both ways: text into taps, represented as dots, and taps back into text. The grid drops K, written as C — the same solution the Polybius square uses to fit 26 letters into 25 cells, only choosing a different pair. A word break appears as a longer pause.

The ingenuity of the system lies in what it requires: nothing. No paper, no light, no elaborate agreed code — just the memorised grid, which fits into a minute of explanation. That is what let it spread among prisoners who had never met, taught knock by knock by those who already knew. It is a rare case of a cipher whose main quality is how easily it travels.