Candidate
A digit (1–9) a cell could still legally hold — one not yet ruled out by anything in its row, column, or 3×3 box. Every empty cell has between one and nine.
A candidate is a digit a cell could legally hold. Every empty cell on a Sudoku grid has between one and nine candidates at any moment, and as placements are made elsewhere, each cell's candidate set shrinks.
How it's derived
A cell's candidates come from a simple subtraction. Start with the digits 1 through 9. Remove every digit already placed in the cell's row. Remove every digit already placed in its column. Remove every digit in its 3×3 box. What remains is the candidate set for that cell. The set only shrinks as the puzzle progresses; it never grows.
Why the term matters
"Candidate" is the technical name for what beginners often just call "the possibilities" for a cell. Every Sudoku technique is, at its core, an argument about candidates. A naked single is the simplest possible argument — "this cell has only one candidate, so the candidate goes there." A hidden single shifts perspective — "this digit has only one candidate cell within this unit." The vocabulary lets harder techniques be described compactly without rewriting the underlying mechanic each time.
For tips on tracking candidates without making the grid unreadable, see Pencil marks without the clutter. For the visual-pattern side of candidate-reading, see Reading pencil marks like a shape.
See also
- Pencil mark— A small handwritten or app-rendered note inside a cell indicating which digits the cell could still legally hold. The visible representation of a cell's candidate set.
- Naked single— A cell on the Sudoku grid that has only one legal candidate left — the simplest deduction in the game, and the one that solves most of an easy puzzle.
- Hidden single— A digit with only one possible cell within a unit (row, column, or 3×3 box) — even if that cell could legally hold other digits. The unit-first sibling of the naked single.
Read more
- Pencil marks without the clutter
When pencil marks help your Sudoku, when they hurt, and the minimal candidate-marking that gets you unstuck without burying the grid.
- Reading pencil marks like a shape
Pencil marks aren't a list of candidates. They're a pattern, and learning to read them as one is the perceptual habit behind every mid-level technique.