Sometimes a cell looks at its row, column, and box, and after all the eliminations, only one number is possible. That is a naked single.
What is it?
A naked single happens when you check all the constraints on a cell and find that eight numbers are already used elsewhere. The remaining number is your answer.
Row uses 5, 3. Column uses 1, 2, 4, 6, 7, 9. Only 8 fits.
How to find them
Pick an empty cell and check what numbers appear in its row, column, and 3x3 box. Cross them off your mental list of 1-9. If only one remains, you found a naked single.
- Choose an empty cell.
- List numbers 1 through 9.
- Remove any number that appears in the same row, column, or box.
- If one number is left, that is your answer.
Worked example
Look at the center cell. The box already has 5, 3, 4, 6, 2, 1, 9, 7. Only 8 is missing. Place it.
All numbers except 8 are used. The center cell is 8.
Tips for spotting them
- Focus on cells where the row, column, and box are already crowded.
- After each placement, nearby cells become easier to solve.
- Combine with last free cell for faster progress.
When to use this technique
Naked singles work throughout the puzzle. Early on, they appear less often. As you fill more cells, they pop up more frequently. Keep checking.