Sudoku Rules & Solving Techniques

One page per technique — what it is, how to spot it, and when it is worth the effort.

Basics

When a row, column or box has eight digits filled, the ninth is decided — no reasoning required.

A digit that cannot go anywhere else in a box belongs in the one cell that is left.

Count what a single cell cannot be. If eight digits are excluded, the ninth is forced.

A cell with exactly one candidate left. Also called a naked single.

A digit that appears as a candidate in only one cell of a unit — even if that cell has other candidates too.

Two cells in a unit with the same two candidates lock those digits out of every other cell.

Two digits confined to the same two cells of a unit, hidden among longer candidate lists.

A digit confined to one row or column within a box can be eliminated from the rest of that line.

Three cells in a unit sharing three candidates between them, in any combination.

A digit restricted to the same two columns in two different rows can be eliminated from the rest of those columns.

The X-Wing pattern with three rows and three columns instead of two.