Sudoku glossary
The words you meet in sudoku guides: unit, peer, candidate, strong link, fish and more.
- Cell
- One of the 81 squares of the grid.
- Row, column, box
- The three kinds of group that must each hold 1 to 9 once.
- Unit
- Any row, column or box.
- Peers
- The 20 other cells that share a row, column or box with a given cell. A digit placed in a cell is removed from all its peers.
- Given (clue)
- A digit printed in the puzzle at the start.
- Candidate (pencil mark)
- A digit that could still go in an empty cell.
- Single
- A digit that is forced in a cell. A naked single is the only candidate of its cell (guide); a hidden single is the only place for its digit in a unit (guide).
- Pair, triple, quad (subset)
- Two, three or four cells in a unit that between them hold exactly that many candidates (naked), or digits that fit only that many cells (hidden).
- Locked candidates
- A digit that, within a box, can only go in one line, or the reverse, so it can be removed from the rest of that line or box (guide).
- Elimination
- Removing a candidate because logic shows it cannot be right.
- Strong link
- Two places for a digit in a unit where it must go in one of them: exactly two candidates, so if one is false the other is true.
- Weak link
- Two places for a digit that see each other: they cannot both be true, but both can be false.
- Fish
- A family of patterns where a digit's candidates in a few rows line up in the same few columns (X-Wing, Swordfish, and larger ones).
- Fin
- An extra candidate that spoils an otherwise perfect fish; a finned fish still allows eliminations (guide).
- Wing
- A pattern of two or three cells with few candidates that force a common digit (XY-Wing, XYZ-Wing, W-Wing).
- Chain, loop
- A sequence of strong and weak links that carries a conclusion from one candidate to another (XY-Chain, X-Chain, nice loops).
- Almost locked set (ALS)
- N cells in a unit with N+1 candidates between them; useful because removing one candidate turns it into a locked set (ALS-XZ).
- Uniqueness
- The rule that a proper puzzle has exactly one solution, used to rule out patterns that would give two (Unique Rectangle).
- Forcing chain, forcing net
- Assume a candidate is true (or false), follow what must follow, and see what holds in every case (guide).
- Backdoor
- A small set of cells that, once solved, lets the puzzle finish with only singles.
How the app writes cells and steps is explained on the notation page.