What is a Naked Single in Sudoku?
A Naked Single is an empty cell with only one legal candidate left after you compare its row, column, and 3×3 box. Because every other digit is already excluded, the remaining candidate is forced and can be placed without guessing.
The technique is called “naked” because the conclusion is visible from the candidates written in that one cell. You do not need to compare the candidate with every other cell in the unit; the cell itself already has a single possibility.
- Check the row for digits that are already used.
- Check the column and 3×3 box for the remaining exclusions.
- Place the digit only when exactly one candidate survives all three units.
How to find Naked Singles quickly
Start with rows, columns, or boxes that already contain many givens. Cells in dense regions usually have fewer legal candidates, so forced cells appear there first. If you use pencil marks, scan for cells that contain a single candidate.
After placing one Naked Single, update the affected row, column, and box immediately. One forced digit often removes a candidate from nearby cells and creates another Naked Single, so the fastest workflow is a repeated scan rather than a one-time search.
Example: one cell, one legal digit
Suppose a cell can see 1, 2, 3, 4, 5, 6, 8, and 9 in its row, column, or box. The only missing legal digit is 7, so the cell must be 7. You do not need a trial branch or a second assumption.
If two candidates remain, it is not a Naked Single. Leave the cell unresolved and continue scanning elsewhere until another placement removes one of those candidates.
What to learn after Naked Singles
When you can find Naked Singles reliably, move to Hidden Singles. The difference is important: a Naked Single is the only candidate inside one cell, while a Hidden Single is the only position for a digit inside a row, column, or box.
Use the Sudoku Solver when you want to check whether the next logical move is a Naked Single. It can identify the technique without filling the entire puzzle first.
Apply it immediately on a real puzzle
A technique becomes automatic only after you use it across many different boards. Play at a manageable level, prioritize certain deductions, and review the step that caused you to get stuck.