Tap Code Converter
Convert plain text into Tap Code knock patterns or decode Tap Code back into readable text.
This classic 5x5 grid system is closely related to the Polybius square and works well for low-tech signaling.
Tap Code places the alphabet into a 5x5 square and represents each letter with two groups of taps: the first group gives the row, and the second group gives the column. For example, A is `.` `.` and T is `....` `....`.
Because the grid only has 25 cells, C and K are usually merged into one position. That is why a Tap Code decoder often returns C when you enter the pattern that could also mean K.
This tool uses `/` between letters and `//` between words so the output stays readable on screen while preserving the same row-column logic used in manual knocking.
Every letter sits at a row and column in this square. To tap a letter, knock the row number first, pause, then knock the column number. So B is row 1, column 2 — one tap, pause, two taps.
| row \ col | 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 | Z |
There are only 25 cells, so K reuses C’s position (row 1, column 3); readers tell them apart from context. This is the same grid the converter above encodes with, so a letter’s taps always match between the chart and the tool.
Take each letter, find its row and column on the chart, and tap the two numbers. Reading W A T E R off the grid:
| Letter | Row, Column | Taps |
|---|---|---|
| W | 5, 2 | ····· ·· |
| A | 1, 1 | · · |
| T | 4, 4 | ···· ···· |
| E | 1, 5 | · ····· |
| R | 4, 2 | ···· ·· |
Put together, WATER taps out as ····· ·· / · · / ···· ···· / · ····· / ···· ··, using a short pause between the two numbers of a letter and a longer pause between letters.
Plain text
MEET
Tap Code
... .. / . ..... / . ..... / .... ....
Plain text
CODE
Tap Code
. ... / ... .... / . .... / . .....
Tap Code is a practical adaptation of the older Polybius square idea, which dates back to classical antiquity. In modern popular culture, Tap Code is most often linked to prisoners passing messages through walls, pipes, or bars when spoken communication is restricted.
Its best-documented use is by American prisoners of war in North Vietnam. It was introduced at Hỏa Lò Prison — the camp the POWs nicknamed the “Hanoi Hilton” — in 1965, when Captain Carlyle “Smitty” Harris recalled the grid from an Air Force survival-training instructor and, with fellow prisoners, spread it so men held in isolation could still communicate. Prisoners often opened with the “shave-and-a-haircut” rhythm to get a neighbor’s attention, then spelled words cell to cell. Over years of captivity the code let them share names, organize, and keep morale up when talking was forbidden.
Unlike Morse Code, Tap Code does not depend on short and long signals. It only needs countable knocks or taps, which makes it useful anywhere you can produce repeated pulses but cannot vary duration reliably.
Authoritative references
Low-Tech Signaling
Useful when only knocks, taps, or other repeated pulses are available.
Cryptography Education
Helps learners understand grid ciphers, coordinate encoding, and constrained alphabets.
Puzzle Design
Common in escape rooms, ARGs, and historical cipher challenges where hidden communication matters.
What is Tap Code?
Tap Code represents each letter by its row and column in a 5×5 alphabet square — you tap the row, pause, then tap the column. It is most associated with knock-based communication, especially among prisoners.
How do you read the tap code chart?
Find the letter in the grid, then tap its row number, pause, and tap its column number. For example WATER is W (5,2), A (1,1), T (4,4), E (1,5), R (4,2).
Why are C and K combined in Tap Code?
A 5×5 square has only 25 cells, so Tap Code merges C and K into one position (row 1, column 3). Readers tell them apart from context.
How do you separate letters and words?
This tool uses a slash between letters and a double slash between words. When tapping by hand, pause slightly longer between letters and longer still between words.
Who used Tap Code in the Vietnam War?
American POWs used it at Hỏa Lò Prison — the “Hanoi Hilton” — after Captain Carlyle “Smitty” Harris reintroduced it in 1965, so prisoners held in isolation could still communicate through cell walls.