Cipher
Tap Code Translator
Tap code, also called the knock code, spells letters as pairs of taps on a 5 by 5 grid. Prisoners of war famously used it to communicate through walls. Type text to encode, or paste taps to decode.
Result appears here
It folds K into C to fit the alphabet into 25 cells. The whole thing runs in your browser.
How the Tap Code works
The 25 letters sit in a 5 by 5 grid (K uses the same cell as C). Each letter is sent as two groups of taps: first the row number, then the column number.
So a letter in row 2, column 3 is tap-tap, pause, tap-tap-tap. This tool shows the taps as dots, with clear spacing between each letter.
Examples
History and origins
Tap code is built on the ancient Polybius square. It became widely known through American prisoners of war in Vietnam, who tapped messages between cells to keep morale and organisation alive.
Its power is that it needs nothing but a wall and a way to make a sound, making it usable when every other channel is cut off.
Frequently asked questions
Why does tap code skip the letter K?
A 5 by 5 grid holds only 25 letters, so K shares a cell with C. Readers infer from context whether a C-cell means C or K, which is rarely ambiguous.
How do you read tap code?
Count the first burst of taps for the row and the second burst for the column, then find that cell in the 5 by 5 grid. This tool decodes it for you automatically.
Learn more
Go deeper on the ideas behind this tool.