How to Solve the Polybius Square Cipher

A grid cipher lays the alphabet out in a square — usually 5×5, with I and J sharing a cell — and describes each letter by where it sits. Some stop there; others shuffle those coordinates or push pairs of letters through the square together.

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How to recognise it

Digits that never go above 5, or letters drawn from a tiny set like A, D, F, G, V and X, mean coordinates. Ciphertext that always comes in an even number of letters points to a cipher that works on pairs.

Polybius Square Each pair of digits is a row then a column in the square. I and J share a cell.

The square

Polybius Square is unsolvable without its square, so it is part of the puzzle rather than a hint — every daily cipher of this kind prints one alongside the message.

A	B	C	D	E
F	G	H	I	K
L	M	N	O	P
Q	R	S	T	U
V	W	X	Y	Z

A worked example

The alphabet sits in a square, and every letter is described by where it lives. Get the square right and the rest is bookkeeping. This one is Polybius Square. The key: 5×5 · I=J. Each pair of digits is a row then a column in the square. I and J share a cell. Look the first pieces up in the square and they come out T R E. The answer was TREASURE MAP.

Working one out from scratch

Draw the square first, keyword and all, and keep it in front of you. Nearly every mistake on a grid cipher is a square built wrong — a repeated keyword letter left in, or J given its own cell.