Polybius Square

Learn

Letters are encoded as coordinate pairs from a 5x5 grid.

The Polybius square is a cipher invented by the Greek historian Polybius around 150 BCE. Letters are arranged in a 5x5 grid, and each letter is represented by its row and column numbers. Since there are 26 letters but only 25 cells, I and J share a position.

Grid: Row 1 (ABCDE), Row 2 (FGHIK), Row 3 (LMNOP), Row 4 (QRSTU), Row 5 (VWXYZ). A=11, B=12, ... I/J=24, ... Z=55

• CAT -> 13 11 44
• HELLO -> 23 15 31 31 34
• DOG -> 14 34 22

How to recognize it

• Two-digit numbers representing coordinates
• First digit is row (1-5), second is column (1-5)
• I and J encode to the same value (24)

Common mistakes

• Confusing row and column order
• Forgetting I/J share a cell
• Not recognizing the 5x5 grid pattern

Practice guidance

Coding & Decoding