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.

Where these words turn up

The first half of this list is the whole vocabulary of an Easy board - singles, scanning, units, givens, and 36 of the 81 cells already filled in. The second half only earns its keep from Expert upward, where 25 givens leave room for fish, chains and uniqueness arguments. Reading a term is one thing; meeting it on a grid is what makes it stick.

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