Naked Single
Also known as Sole Candidate.
A cell whose row, column, and box between them already contain eight different digits has only one candidate left — and that digit must go in.
Row 5 already has 2, 6, 8, and 9 placed — four digits ruled out.
How to spot it
- 1List the digits 1–9 for the empty cell.
- 2Strike out every digit already present in its row, column, or box.
- 3If a single digit remains, place it.
When to use
Constantly — it's the final step that turns most deductions into an actual placement.
When not to
Don't hunt for naked singles blindly on a bare grid; they emerge after you've pruned candidates elsewhere.
Common mistake
Filling a naked single but forgetting to remove that digit from its peers, leaving stale candidates behind.
Worked Example
Take an empty cell in the center of a puzzle in progress. Its row already has 2, 6, 8, and 9 placed — four digits ruled out before the column or box even get involved. Its column adds 1 and 5 to the ruled-out list, and the center 3×3 box contributes 7 and 3.
Between the three of them, that's eight different digits accounted for — 1, 2, 3, 5, 6, 7, 8, and 9 — leaving exactly one candidate for that cell: 4. No single row, column, or box did all the work alone; together they left no other option. That's what makes a naked single "naked" — once you write out the candidates, only one is left standing.
From the Blog
- Sudoku for Beginners: How to Solve Your First Puzzle, Step by StepNew to Sudoku? Learn the three rules, your first moves, and solve an easy puzzle step by step — with worked grid diagrams and no experience needed.
- The 5 Sudoku Techniques Every Beginner Should Learn FirstSkip the overwhelm. Master these five techniques — scanning, naked singles, hidden singles, cross-hatching, and pointing pairs — with worked grid diagrams for each.