Type a message to get knocks, or paste dots or numbers to read them.
Tap code puts the alphabet in a 5×5 grid. There are 26 letters and 25 cells, so C and K share a cell (in the Polybius square, I and J share instead). Each letter is two groups of knocks: the row number, a short pause, then the column number. H is row 2, column 3:
•• •••, or knock-knock … knock-knock-knock.
A longer pause separates letters. Experienced tappers shortcut a lot, for example one knock on its own for “yes” and a few common abbreviations, but the grid is all you need to read a message.
American prisoners of war in North Vietnam used tap code to keep each other alive and sane through cell walls for years. It spread from cell to cell, taught by tapping, and is credited with holding the prisoners' chain of command together. It's easier to learn than Morse because there's nothing to memorise beyond the grid, and a tired or injured person can still get it right.
Puzzles write tap code as dots (•• •••), as runs of any character (xx xxx), or as numbers (23). The decoder reads all three. Separate letters with / and words with //, or just use bigger gaps. Press Play knocks to hear it at a steady pace.
Letters sit in a 5×5 grid with C and K sharing a cell. Each letter is two groups of knocks: first the row, then the column. H is row 2, column 3: knock knock, pause, knock knock knock.
Twenty-six letters don't fit in a 25-cell grid, so K is tapped as C. Context makes it obvious, and the tool decodes it as C.
It's best known from American prisoners of war in Vietnam, who used it to talk through cell walls. The grid itself goes back to the Polybius square of ancient Greece.