The tap code is knocked on a wall or a pipe: each letter is given by two series of knocks, the row then the column in a 5 × 5 square (K is knocked as C). A longer pause separates letters. Load a recording: the tool counts the knocks, measures the pauses and reads the message. Everything happens in your browser.
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The square
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C/K | D | E |
| 2 | F | G | H | I | J |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
In real life: the tap code was used by American prisoners of war in Vietnam; it derives from the Polybius square. By ear, write down the groups of knocks in pairs (3-1 = L, 1-1 = A…). Unlike Morse, all knocks are identical: only their number counts.