Key Points
- A candidate is any digit still legally possible in a cell
- Units are the 27 groups (9 rows + 9 columns + 9 boxes) that each must hold 1–9
- A naked subset has N candidates shared by exactly N cells in a unit
- A hidden subset has N cells that are the only places N candidates can go in a unit
- Mastering this vocabulary lets you follow advanced guides and improve faster
Basic Grid Terms
| Term | Definition |
|---|---|
| Cell | A single square in the 9×9 grid |
| Row | A horizontal line of 9 cells |
| Column | A vertical line of 9 cells |
| Box / Block / Nonet | One of the nine 3×3 sub-grids |
| Unit | Any row, column, or box — 27 total |
| Given / Clue | A digit printed in the puzzle at the start |
| Candidate | A digit that could legally go in an empty cell |
Pencil Mark Terms
Pencil marks (or candidates) are the small digits written in empty cells to track which values are still possible. Eliminating a candidate means removing it because the same digit appears in a shared unit. When a cell has only one candidate left, that is its solved value.
Read the full guide on using notes in Sudoku for how to manage pencil marks efficiently.
Subset Terms
| Term | What It Means |
|---|---|
| Naked Single | A cell with exactly one candidate remaining |
| Hidden Single | A candidate that can only go in one cell within a unit |
| Naked Pair | Two cells in a unit sharing the same two candidates |
| Hidden Pair | Two candidates confined to the same two cells in a unit |
| Naked Triple / Quad | N cells sharing exactly N candidates in a unit |
| Hidden Triple / Quad | N candidates confined to exactly N cells in a unit |
Intermediate and Advanced Terms
| Term | What It Means |
|---|---|
| Locked Candidates | Candidates in a box restricted to one row or column (or vice versa), allowing cross-unit eliminations |
| X-Wing | A digit forming a rectangle across two rows and two columns; eliminates candidates outside the pattern |
| Swordfish | Like X-Wing but across three rows and three columns |
| Conjugate Pair | Two cells in a unit where a digit can only go in one of them |
| Strong Link | A connection between two cells where a digit must be in one of them |
| Weak Link | A connection where a digit may be in one cell but doesn't have to be |
Where to Learn Each Technique
Every term above has a dedicated page in the techniques library. Start with Naked Singles and Hidden Singles — those two techniques will solve most Easy and Medium puzzles. Then work through pairs and locked candidates as you progress to Hard and beyond.