Calculate the cell’s candidates
Choose an empty cell and collect the digits already used in its row, column, and box. Remove those digits from 1 through 9. If only one digit survives, that digit must occupy the cell. For example, if the three units together already contain 1, 2, 3, 4, 6, 7, 8, and 9, then 5 is the only legal candidate.
The proof is local but complete: placing any other digit would immediately duplicate a number in at least one of the three units. You do not need to know the rest of the solution to make the move confidently.
Verify before placing
Most naked-single mistakes come from overlooking a digit in one unit or working with stale notes. Scan the row from left to right, the column from top to bottom, and all nine cells of the box. If you use notes, confirm that candidates removed by recent placements are no longer shown. A ten-second verification is cheaper than unwinding several incorrect moves.
Exploit the cascade
After placing a naked single, remove that digit from candidates in the same row, column, and box. This can turn a neighboring two-candidate cell into another naked single. Easy puzzles often contain long cascades where each confirmed placement creates the next forced move. Rescan the affected units before jumping elsewhere in the grid.
- Find an empty cell with a short candidate list.
- Confirm every other digit is excluded by a peer.
- Place the sole remaining candidate.
- Update the twenty peer cells that share its row, column, or box.
- Look for a new single created by the update.
Distinguish naked and hidden singles
A naked single has one candidate in the cell. A hidden single may sit in a cell with several candidates, but one of those digits has no other possible location in the unit. Both are forced placements, yet the proof is framed differently. Learning to switch between cell scanning and unit scanning prevents you from getting stuck after the obvious naked singles are gone.
Frequently asked questions
Can a naked single be wrong?
Not when the candidates were calculated correctly from a valid grid. An error indicates a missed peer, a stale note, or an earlier incorrect entry.
Why does a naked single sometimes appear after another move?
A new placement removes its digit from peer candidates. A nearby cell that previously had two possibilities may then be reduced to one.
Should I fill every naked single as soon as I see it?
Usually yes, after verification. Naked singles are forced placements, and resolving them early often simplifies peer cells before you need harder techniques.