Sudoku Glossary
Every sudoku term you'll meet, in plain English. Where a term is a solving technique, it links to a full guide with a verified worked example.
The board
- Grid
- The 9×9 playing area, made of 81 cells.
- Cell
- A single square that holds one digit from 1 to 9.
- Box (region)
- One of the nine 3×3 blocks. Also called a region or block.
- Unit
- Any row, column, or box — the three groups a digit must appear in exactly once.
- Given (clue)
- A digit printed at the start of the puzzle. A classic sudoku needs at least 17 givens to be unique.
- Candidate
- A digit that could still legally go in a cell. Tracked with pencil marks.
- Pencil marks
- Small candidate notations written in a cell to track possibilities.
- Bivalue cell
- A cell with exactly two candidates — the building block of wing and chain techniques.
Basic techniques
- Naked single
- A cell with only one candidate left, so that digit must go there.
- Hidden single
- A digit that can go in only one cell of a unit, even if that cell has other candidates.
- Scanning
- Checking each unit for where a digit can legally go — the core beginner skill.
- Naked pair
- Two cells in a unit with the same two candidates, letting you erase those digits from the rest of the unit.
- Hidden pair
- Two digits confined to the same two cells, letting you erase every other candidate from those cells.
- Naked/hidden triple
- The three-cell versions of pairs.
Intermediate techniques
- Pointing pair
- A candidate confined to one line within a box, eliminating it from the rest of that line.
- Box-line reduction
- A candidate confined to one box within a line, eliminating it from the rest of the box.
- Locked candidate
- The umbrella term for pointing pairs and box-line reduction.
Advanced techniques
- X-wing
- A single digit locked into a 2×2 rectangle across two rows and two columns.
- Swordfish / Jellyfish
- The 3×3 and 4×4 generalisations of the X-wing.
- XY-wing
- A pivot bivalue cell and two pincers that force a shared candidate to be eliminated.
- Coloring
- Two-colouring a digit's conjugate pairs to find contradictions and eliminations.
- Conjugate pair
- A unit where a candidate appears in exactly two cells — a strong link.
- Forcing chain
- A chain of implications that reaches the same conclusion from every starting assumption.
- Nishio
- A single-candidate trial: place it, follow the forced moves, and eliminate it if a contradiction appears.
- Unique rectangle
- A uniqueness technique that avoids a four-corner 'deadly pattern' of two candidates.
- Deadly pattern
- A configuration that would give two solutions — forbidden in a proper puzzle, and exploitable.
- BUG (Bivalue Universal Grave)
- A near-all-bivalue state; the one three-candidate cell resolves the puzzle.
Frequently asked questions
What is a candidate in sudoku?
A candidate is a digit that could still legally go in a particular cell — one that doesn't already appear in that cell's row, column or box. Solvers track candidates as small pencil marks, and most techniques work by eliminating candidates until one remains.
What is a unit in sudoku?
A unit is any row, column, or 3×3 box. Each of the digits 1–9 must appear exactly once in every unit, and that constraint is the basis of every solving technique.
What does bivalue mean in sudoku?
A bivalue cell is a cell with exactly two candidates. Bivalue cells are the building blocks of advanced techniques like XY-wings, coloring and forcing chains.
What is a deadly pattern in sudoku?
A deadly pattern is a configuration — most simply, four cells forming a rectangle that all share the same two candidates — that would allow two valid solutions. Because a proper sudoku has exactly one solution, the pattern is impossible, and uniqueness techniques exploit that to eliminate candidates.