The Tap Code
Every letter is two numbers: a row and a column on a five-by-five grid, and each number is that many knocks. There is no long and short, and that is the point of it.
The grid
| 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 |
How to send a letter
- Find your letter in the grid: C and K share a cell, so K is sent as C.
- Knock the row number, one to five taps.
- Pause, then knock the column number.
- Pause longer between letters, and longer again between words.
Worked example: WATER
| Letter | Row, column | Knocks |
|---|---|---|
| W | 5, 2 | ..... .. |
| A | 1, 1 | . . |
| T | 4, 4 | .... .... |
| E | 1, 5 | . ..... |
| R | 4, 2 | .... .. |
Why it beats Morse through a wall
Morse encodes meaning in duration: a dash is a signal held three times as long as a dot. That works over a wire, a radio, a lamp, any channel that can hold a signal steady for a measurable time. It does not work when your only instrument is a fist and a concrete wall, because a knock has no length. It happens, and then it has happened.
The tap code throws duration away and puts everything in counting and grouping instead. That costs speed: the average letter needs five or six taps where Morse averages under three elements, but it buys a code that works on any channel capable of a noise and a silence. Prisoners have independently arrived at some version of it for at least two centuries, which is a strong argument that it is the natural answer to the problem.
It has one more property worth noticing: it is easy to teach through the wall itself. A new arrival who does not know the code can be taught it by someone tapping the alphabet slowly, because the system is just counting. Teaching Morse the same way would require conveying the idea of a dash first.
Frequently asked
Why is there no K?
A 5x5 grid holds 25 cells and the alphabet has 26 letters, so one has to go. K is the one dropped because it is rare and because C stands in for it readably, "back" tapped as "bacc" causes nobody any trouble. Some versions drop the far rarer Q instead and send it as K.
How is it different from Morse code?
Morse needs a channel that can carry two different lengths of signal. The tap code needs only a channel that can make a noise, because every symbol is a count of identical taps. That is a much lower bar, and it is the entire reason the code exists: you cannot send a dash through a wall, but you can knock twice.
Is it slower than Morse?
Considerably. The average letter takes about five and a half taps plus two pauses, where Morse averages under three elements. The tap code was never competing on speed, it was competing with having no way to communicate at all.
Where was it actually used?
Most famously by American prisoners of war in Vietnam, where it spread through Hoa Lo prison after 1965 and let men in isolation hold conversations for years. It is much older than that: a version of the Polybius square dates to antiquity, and Russian prisoners used a similar grid in the nineteenth century, but the Vietnam use is why it is remembered.