The tap code (also called the knock code) is a simple way to send letters as taps or knocks, made famous by American prisoners of war who used it to communicate through cell walls. Instead of showing the usual number pairs, this tool draws each letter as two clusters of dots so you can see exactly how many times to tap. Everything runs locally in your browser.
How it works
Tap code lays the alphabet into a 5×5 grid, dropping one letter so it fits 25 cells. The conventional grid omits a dedicated K and reuses the C cell for both:
1 2 3 4 5
1 A B C 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 ZTo encode a letter you find its row and column. For example H sits at row 2, column 3, so it is tapped as tap tap … tap tap tap — a group of 2 dots, a pause, then a group of 3 dots. This tool renders the first count and the second count as separate dot groups so the rhythm is obvious at a glance.
Worked example: encoding WATER
The word WATER breaks down as follows:
| Letter | Row | Column | Taps |
|---|---|---|---|
| W | 5 | 2 | ●●●●● pause ●● |
| A | 1 | 1 | ● pause ● |
| T | 4 | 4 | ●●●● pause ●●●● |
| E | 1 | 5 | ● pause ●●●●● |
| R | 4 | 2 | ●●●● pause ●● |
A word boundary is shown as a slightly longer pause between the letter groups.
Historical background and why the C/K merge exists
Tap code was reportedly developed and popularized among American prisoners of war held in North Vietnam in the 1960s and 1970s. The 5×5 Polybius square only has 25 cells, and the 26-letter English alphabet does not fit. The conventional solution is to share the C cell between C and K, since in many words the intended letter is inferrable from context — “cat” and “back” work fine; only rare ambiguous combinations cause confusion.
Some users of the cipher used I and J merged instead, following older Polybius conventions. This tool follows the standard American POW convention of merging C and K.
Practical uses today
- Puzzle design. Escape rooms and puzzle hunts frequently use tap code because the 5×5 grid is elegant and learnable, and the dot-visual format makes a satisfying physical challenge.
- Learning cryptography. Tap code is an ideal first cipher for students because it is visual, requires no mathematics, and has a compelling historical narrative behind it.
- Morse code comparison. Unlike Morse, every tap code letter uses exactly two groups of taps, which makes decoding predictable and removes the ambiguity of variable-length Morse sequences.
Because K is not in the grid, type C instead and let the reader infer the intended letter from context. Spaces and punctuation are shown as gaps so you can keep word boundaries when tapping a longer message.