Naked singles in Sudoku: find the one possible digit
Look inside one empty cell. Remove every digit already used in its row, column, or 3×3 box. When only one digit survives all three checks, the placement is certain.
Definition
A naked single is an empty cell with exactly one legal candidate after its row, column, and 3×3 box are checked.
When to use it and what to verify
When to look for one
After candidate notes are current, inspect cells with the fewest remaining candidates. Recheck after every nearby placement.
Required condition
Exactly one digit must remain legal after the cell’s complete row, column, and 3×3 box are checked.
Common mistake
A cell is not a naked single merely because only one note was written. Missing or stale notes can create a false single.
One cell has one legal candidate.
- 1. Check the rowRow 1 rules out 3, 5, and 6.
- 2. Check the column and boxColumn 3 rules out 2, 8, and 9. The top-left box rules out 1 and 7.
- 3. Place the survivorOnly 4 remains at R1C3. This forced placement is called a naked single.
Digits 1, 2, 3, 5, 6, 7, 8, and 9 are blocked at R1C3. Only 4 remains, so R1C3 = 4.
A reference verified by Doku.su
Doku.su checks every placement or elimination in this example with its deterministic Sudoku engine. Illustrative candidates are labelled when they do not represent a complete puzzle state.
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