Naked Single
Only one candidate digit remains possible in a cell.
Also known as: Sole Candidate, Forced Digit, Singleton
Quick Summary
A Naked Single occurs when 8 of the 9 digits are already present in the combined row, column, and 3×3 box seeing a cell, leaving exactly one digit mathematically possible.
Understanding the Naked Single Technique
The Naked Single is the bedrock of Sudoku solving. It is called 'naked' because once candidates are penciled into the cell, the digit sits completely exposed with no other contenders. While simple in concept, discovering naked singles on an unpenciled board requires scanning all three intersecting houses (row, column, and box) simultaneously. Puzzles in beginner and gentle tiers resolve predominantly through cascades of naked singles.
Visual Examples
Green cells highlight the Naked Single pattern, amber cells denote where candidates are affected, and struck-through pencil marks show the eliminations. Use the buttons or arrow keys to browse examples.
How to Spot & Solve a Naked Single (Step-by-Step Guide)
The complete visual scanning and logical deduction process to execute this technique:
- 1Select an unsolved cell on the grid.
- 2Check all placed digits along its row, down its column, and inside its 3×3 box.
- 3If 8 distinct digits are already accounted for across those intersecting units (e.g. 1 through 5, and 7 through 9), the missing number (6) is the only legal option left.
- 4Confidently place that single remaining number into the cell.
Common Traps & Mistakes to Avoid
- Overlooking one of the 3 intersecting houses—often checking the row and column carefully but forgetting a digit already placed in the opposite corner of the 3×3 box.
- Confusing a Naked Single with a Hidden Single: a Naked Single is forced because the *cell* has only one choice; a Hidden Single is forced because the *house* has only one place for a digit.
- Prematurely placing a digit based on intuition without verifying that all other 8 digits are genuinely accounted for.
Frequently Asked Questions
What is the difference between a Naked Single and a Hidden Single?
A Naked Single means a specific cell has only one possible candidate remaining across its row, column, and box. A Hidden Single means a digit has only one legal cell inside an entire row, column, or box, even though that cell might still hold multiple other candidates.
Do I need full pencil marks to find Naked Singles?
Not necessarily. Skilled solvers find naked singles on unpenciled boards by scanning cells whose row, column, and box share many placed digits. However, writing candidate pencil marks makes naked singles immediately obvious as cells with only one number.
Can an expert Sudoku puzzle contain Naked Singles?
Yes! Even the most diabolical puzzles often finish with a cascade of naked singles once an advanced technique (like an X-Wing or ALS Chain) makes key eliminations and cracks the puzzle open.
Related Solving Techniques
Expand your repertoire by learning prerequisites and advanced counterparts:
Ready to Practice Naked Single?
Try a puzzle in the Beginner tier (Peak Score 1–10), or browse all 25 techniques.