Polybius Square & Tap Code Cipher
Coordinate ciphers from ancient Greece to POW camps.
About this tool
The Polybius square maps each letter to a pair of coordinates in a 5×5 grid, turning the alphabet into pairs of small numbers — the basis of many later ciphers. Tap code is its acoustic descendant: rows and columns tapped out on cell walls, famously used by American POWs in Vietnam.
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Frequently asked questions about Polybius & Tap Code
What is a Polybius square?+
A 5×5 grid that maps each letter to a pair of coordinates (row, column), turning letters into number pairs. It underlies many later ciphers.
What is tap code?+
An acoustic version of the Polybius square where coordinates are tapped out — famously used by American POWs in Vietnam to communicate through cell walls.
Why do I and J (or C and K) share a cell?+
A 5×5 grid holds 25 letters, one short of 26, so two letters share a cell — I/J in the classic Polybius square, C/K in tap code.
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