Sudoku Glossary

The terms below appear throughout our strategy guides. Each one links here when first used in an article — jump to a definition and use your browser’s back button to return to where you were reading.

Region

A region is a group of nine cells that must contain each of the digits 1–9 exactly once. In classic Sudoku, every row, every column, and every 3×3 box is a region. Variants may add extra regions — for example, the two long diagonals in Diagonal Sudoku, or the irregular shapes in Jigsaw Sudoku — and the same rule applies: each region holds 1–9 with no repeats.

Box

A box is one of the nine 3×3 sub-grids that partition a classic Sudoku grid. A box is a region in its own right: every digit from 1 to 9 must appear in it exactly once.

See

Two cells see each other if they share at least one region — i.e. they are in the same row, the same column, the same box, or the same variant-specific region. Because two cells that see each other belong to the same region, they cannot hold the same digit. This single idea underpins almost every elimination technique.

A strong link between two candidates of the same digit means exactly one of them must be true. The simplest case: a digit appears as a candidate in only two cells within a region — one of those two cells must be the solution. If you can rule one out, the other is forced.

Strong links are the backbone of chaining techniques such as X-Wing, Skyscraper, Two-String Kite, Simple Coloring, X-Cycles, and 3D Medusa.

A weak link between two candidates means they cannot both be true — but, unlike a strong link, both can be false. Any two candidates of the same digit that see each other form a weak link.

Many advanced strategies (Simple Coloring, X-Cycles, XY-Chain, 3D Medusa) work by following a chain that alternates strong and weak links: along such a chain, every other candidate must be true, so the chain’s endpoints together rule out anything that sees both.