Tap Code Translator (Prisoner’s Knock Code)
Translate text to and from tap code as dots, number pairs or tap words, using the 5×5 grid where C and K share a cell. Decoding accepts any separators.
| 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 |
Tap code sends letters as two groups of knocks: the first gives the row, the second the column of a 5×5 grid. American prisoners of war in Vietnam famously used it to talk through cell walls. Because the grid has 25 cells, K is replaced by C. This translator writes text as dots, number pairs or tap words and decodes any of them back, tolerating different spacing and separators.
How to use this tool
- Choose text → tap code or tap code → text.
- Type the message, or paste the code (dots, digits like 2-3 or 23, or the word "tap").
- Pick how the code should be written when encoding.
- Separate words with "/" when you decode several words.
Formula
letter → (row, column) in the grid ABCDE / FGHIJ / LMNOP / QRSTU / VWXYZ, sent as row taps, a pause, then column taps
The grid is the Polybius square with K left out; C stands for both C and K. Letters are separated by a longer pause, which is written here as two spaces (dots) or a space (numbers). In practice X was often tapped to end a sentence, and common words were abbreviated.
Example
HELLO is H = row 2, column 3 (.. ...), E = 1, 5 (. .....), L = 3, 1 twice and O = 3, 4, so the dots read .. ... . ..... ... . ... . ... .... — or 23 15 31 31 34 as digit pairs. "Hello world" needs 55 taps in total.
Frequently asked questions
Why is there no K?
Five rows of five hold 25 letters, so one has to go. Tap code drops K and uses C instead, because the meaning is almost always clear from the word (cnow = know).
How is tap code different from Morse code?
Morse needs two different signals (short and long). Tap code needs only identical knocks and pauses, so it works with anything that makes a noise — a pipe, a wall, a cup.
Is it the same as the Polybius square?
It is a Polybius square without K and with the coordinates sent as taps. The Polybius tool uses I/J sharing instead, so the two give different numbers for letters after J.
Last updated:
Page address with these inputs: https://kakao.mobile/tools/tap-code-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). Tap Code Translator (Prisoner’s Knock Code). https://kakao.mobile/tools/tap-code-tool/
“Tap Code Translator (Prisoner’s Knock Code).” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/tap-code-tool/.
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