Polybius Square Cipher
Encode and decode messages using the Polybius square — each letter is replaced by its row and column number in a 5×5 grid.
What Does This Tool Do?
The Polybius square arranges the alphabet in a 5×5 grid (I and J share a cell). Each letter is encoded as its row and column number — so A=11, B=12, Z=55. A Greek and Roman cipher used since antiquity.
Key Features
5×5 Grid
Classic alphabet square (I=J combined).
Number Pairs
Each letter becomes a 2-digit coordinate.
Both Directions
Encode and decode.
Copy Result
One-click copy.
How to Use
- Enter your message.
- Click Encode to get row-column number pairs.
- To decode, enter number pairs separated by spaces and click Decrypt.
Frequently Asked Questions
What is the Polybius square?▾
A 5×5 grid holding the alphabet (I and J share one cell). Each letter is identified by (row, column): A=11, B=12, C=13, D=14, E=15, F=21, etc.
Why do I and J share a cell?▾
The Latin alphabet has 25 letters when I and J are combined (they were originally the same letter). Modern English uses 26 letters, so one cell holds two.
What are its historical uses?▾
The Polybius square was used in ancient Greece for long-distance signalling (torch semaphore), and it forms the basis for the Nihilist and ADFGVX ciphers.