A Naked Single is an unsolved cell with exactly one legal candidate left.
The proof is entirely cell-first:
Every other digit is blocked by the cell's row, column, or box.
If only 8 survives those checks, the cell is 8. You do not need to compare where 8 appears elsewhere in the unit.
Why is it called a Naked Single?
The word single means there is one forced possibility.
The word naked means that possibility is visible directly in the cell's candidate set: there are no other candidates “hiding” it.
A cell with:
{8}
is a Naked Single.
By contrast, a cell with {2, 6, 9} could still contain a Hidden Single if 6 is the only place for digit 6 in the row, column, or box.
How a Naked Single works
For any unsolved cell, begin with digits 1–9.
Remove digits that already appear in:
- the same row;
- the same column;
- the same box.
If eight digits are ruled out, the ninth digit is the only legal candidate.
That candidate must be placed.
There is no guess and no preference between alternatives because no alternative remains.
Step-by-step Naked Single example
How to spot Naked Singles with pencil marks
If full candidates are displayed, look for cells containing a single small digit.
For example:
r4c2 {7}→ place 7;r8c9 {3}→ place 3.
This is one reason automatic candidate mode makes Naked Singles visually easy.
After placing the digit, remove it from all unsolved peers and check whether another cell has become a Naked Single.
How to spot Naked Singles without pencil marks
On paper or with notes hidden, inspect cells whose peers already contain many different digits.
A useful process is:
- choose an empty cell;
- list the digits present in its row;
- add digits present in its column;
- add digits present in its box;
- if those peers account for eight different digits, the missing digit is forced.
This can take more visual work than reading a one-candidate note, which is why the apparent difficulty of Naked Singles depends partly on the interface.
Naked Single vs a last missing digit
A row, column, or box with one empty cell also gives a forced placement: the missing digit must fill that cell.
That explanation starts from the unit.
A Naked Single starts from the cell's candidate set.
The same placement can sometimes be obvious from more than one viewpoint. Use the shortest correct explanation.
Naked Single vs Hidden Single
These are both forced placements, but the viewpoint differs.
Naked Single
This cell has only one possible digit.
Hidden Single
This digit has only one possible cell in the unit.
A Hidden Single cell can still have several candidates. A Naked Single cannot.
Why one Naked Single can create another
Suppose cell A has {4, 7} and a peer is solved as 4.
Candidate 4 is removed from cell A, leaving:
{7}
Cell A is now a Naked Single.
Placing 7 can then remove 7 from its peers and create another forced cell.
Easy Sudokus often contain long chains of this simple propagation.
Common Naked Single mistakes
Placing a digit because it “looks likely”
A Naked Single requires exactly one legal candidate. If two remain, the cell is not solved by this technique.
Forgetting the box
Row and column checks are not enough. A candidate can also be blocked by the 3×3 box.
Using stale notes
A cell may appear to have two candidates only because one impossible note was never removed. Update candidates after placements.
Confusing a Hidden Single with a Naked Single
If the cell has several candidates but one digit is unique in the unit, that is Hidden Single logic.
Tips for finding Naked Singles
- With notes visible, scan for one-candidate cells after every elimination.
- Without notes, inspect cells in dense areas of the grid.
- After placing a digit, check its row, column, and box immediately.
- If no Naked Single exists, do not force one; switch to unit-focused Hidden Singles or another technique.
FAQ
Is a Naked Single the easiest Sudoku technique?
It is one of the most basic techniques, especially when candidates are displayed. A last-missing-digit placement can be even more visually immediate.
Can a Naked Single have two candidates?
No. By definition, the cell has exactly one remaining candidate.
Do I need to write candidates to find a Naked Single?
No, but visible candidates make the pattern easier to spot. You can derive the same result by checking all peer digits manually.
Is a Naked Single always safe to place?
Yes, if the candidate set is correct. The only remaining candidate is forced by the Sudoku constraints.
What to learn next
Learn Hidden Singles next. The two techniques teach the two fundamental ways to find a forced placement: one possibility for a cell, or one position for a digit in a unit.