Sudoku Glossary

Sudoku has a vocabulary all its own, and solving guides throw it around freely. Here's every term you'll meet, defined plainly — with a link to the technique that puts it to work. Skim it once and the rest of the Sudoku School reads a lot easier.

The grid

• Cell. One of the 81 squares. Each ends up holding a single digit 1–9.
• Unit (or house). Any row, column or 3×3 box. Every unit must contain each digit exactly once — the one rule the whole game rests on.
• Box. One of the nine 3×3 blocks the grid is divided into. Also called a region.
• Given (or clue). A digit filled in at the start. The fewer givens, the harder the puzzle — the minimum is 17.

Candidates and marks

• Candidate. A digit that could still legally go in a cell. The heart of almost every technique.
• Pencil marks. The small candidate numbers you note in an empty cell — see the pencil marks guide.
• Bi-value cell. A cell with exactly two candidates. The building block of wings and chains.
• Snyder notation. A tidy marking system that notes a digit only where it has two homes in a box — see Snyder notation.

Naked and hidden

• Naked. A pattern you see directly in the candidates — e.g. a naked pair is two cells showing only the same two candidates.
• Hidden. A pattern found by tracking where a digit can go — e.g. a hidden single is a digit with only one home in a unit, even amid other candidates.
• Single. A cell that can only be one digit (naked single) or a digit with only one home (hidden single) — either way, place it.
• Subset. A pair, triple or quad — two, three or four cells sharing the same small set of candidates.

Advanced patterns

• Intersection (locked candidate). When a digit is trapped where a box and a line cross — see pointing pairs and box/line reduction.
• Fish. A single-digit pattern across N lines and N cross-lines — the X-Wing (2), Swordfish (3) and Jellyfish (4).
• Wing. A three-cell pattern that forces a digit out of a fourth cell — the XY-Wing is the classic.
• Deadly pattern. A configuration that would give a puzzle two solutions, and so is forbidden — the basis of the Unique Rectangle.
• Conjugate pair. A digit with exactly two possible cells in a unit — the link that chains and colouring techniques travel along.

Killer and variants

• Cage. A dashed-outline group of cells in Killer Sudoku with a target sum; digits in a cage never repeat.
• Rule of 45. Every unit sums to 45 — the foundation of Killer, see the rule of 45.
• Innie / outie. A single cell poking into or out of a region, pinned down by the sums — see innies and outies.
• Variant. Any Sudoku with an extra rule — Killer, X-Sudoku, Samurai and many more.