A naked single is an empty cell with exactly one legal candidate. Once every other digit is ruled out by its row, column or 3 × 3 box, you can place the remaining digit without guessing. The useful part is learning to check all three constraints, even when a cell looks almost empty.
What counts as a candidate?
Start with digits 1–9. Remove every digit already placed in the cell’s row, column and box. The digits left are its candidates. A candidate is a possibility, not a promise: another deduction may eliminate it later.
For a naked single, this complete list has length one. A cell displaying one pencil note is not necessarily a naked single: you may simply have omitted other legal candidates.
A naked single, step by step
This hand-built teaching example is separate from Puzzuno’s Daily puzzles. Consider row 1 below. The highlighted first cell is empty; the other empty cell is column 3.
- Check the row. It contains 2, 4, 5, 6, 7, 8 and 9. The missing digits are 1 and 3. Either empty cell could contain either digit if you look only at the row.
- Check the selected cell’s column. Column 1 contains 4, 7, 2, 5, 8, 3, 6 and 9 in rows 2–9. In particular, the 3 in row 7 rules out 3 for the selected cell.
- Check its box. The top-left box contains 2 in row 1, 4, 5, 6 in row 2, and 7, 8, 9 in row 3. It contains no 1, so 1 is still allowed.
- Place 1. The complete candidate list is now just {1}. Row 1 then needs 3 in column 3, giving a second certain move.
The row alone did not settle the first cell. Combining the row and column did; checking the box confirms the placement does not conflict. These example givens are compatible with a completed Sudoku, rather than unrelated numbers chosen to force an answer.
Use singles as a repeatable solving routine
Scan cells where many neighbors are already filled. Work out their complete candidates and place any singles you find. After placing a number, revisit empty cells in that row, column and box: the new placement can turn two candidates into one.
You do not need to write notes in every cell before starting. A short mental check is enough in a crowded area. For a less obvious cell, use pencil notes and keep them complete. On Puzzuno, use Notes to enter your possibilities, then turn Notes off to place a confirmed digit. See the Sudoku controls and How to Play guide for the full workflow.
If there are no singles, stop repeating the same scan. Look for another deduction and come back after the board changes. A puzzle requiring more advanced logic is not evidence that the naked single technique is wrong.
Common mistakes
- Checking only the row. A number missing from a row can still be blocked by the column or box.
- Trusting incomplete notes. Recompute the candidates before treating a lone handwritten note as proof.
- Keeping stale candidates. Remove a digit from affected notes after a confirmed placement; then look for new singles.
- Ignoring a contradiction. Zero candidates means something is wrong with the current entries or reasoning. Review earlier moves instead of choosing an arbitrary number.
Frequently asked questions
How is a naked single different from a hidden single?
A naked single asks which digits can go in one cell. A hidden single asks which cells can hold one particular digit within a row, column or box. That digit may have only one location even when the cell still has several candidates.
Does a naked single depend on the puzzle having a unique solution?
No. If the current placements are correct, the row, column and box rules force the cell’s only legal candidate. You do not need to assume uniqueness to make this deduction.
Can I solve every Sudoku with naked singles?
No. Some puzzles require other techniques. Use singles whenever they appear, but do not turn a two-candidate cell into a guess just to keep moving.
Try the technique on your next puzzle
Open Sudoku practice and find one empty cell with a crowded row or column. Explain each elimination before entering the answer. When you are ready for a shared daily puzzle, visit today’s Daily Challenges.