What it is
A naked single is a cell where every candidate except one has been eliminated by the digits already present in its row, column and box. The remaining digit is certain and can be placed immediately.
How to spot it
Scan cells whose row, column and box are already crowded. Count which digits 1–9 are still legal; if only one survives, place it. With full pencil marks, naked singles are simply the cells showing a single note.
Interactive example
Use Next to walk through the deduction on the board.
Step by step
- We want the centre cell. Its row already contains 1, 2, 3 and 4 — none of those can repeat.
- Its column adds 5, 6 and 7 to the exclusions.
- And the centre box already holds an 8.
- Eight digits are excluded — only 9 remains. That's a naked single.
Common mistake: Confusing it with a hidden single. A naked single is about one cell having one candidate. A hidden single is about one digit having one possible cell in a unit — the cell itself may show several notes.
Put it into practice
Examples on this page are validated with the Play Sudoku Daily puzzle engine.