Naked Single Technique: The Most Direct Sudoku Solving Method
Naked Single, also known as "Sole Candidate" or "Forced Digit", is the most basic and direct solving technique in Sudoku. The core concept is simple: when a cell has only one remaining candidate, that number is the answer.
Sudoku rules require each cell to contain a digit from 1-9, with no repetition in the same row, column, or box. When elimination reveals that a cell can only contain one specific number, that number is the definite answer.
Before reading this article, we recommend understanding Sudoku naming conventions.
Why Is It Called "Naked"?
The Naked Single is called "naked" because the sole candidate is directly visible—you only need to look at the cell itself to see there's only one number left, without analyzing the entire row, column, or box. The answer is "naked" right there!
This contrasts with Hidden Single: Hidden Singles require examining the entire region to discover that a number can only go in one position, as the candidate is "hidden" among multiple possibilities.
Example: The Sole Candidate in R1C9
Let's look at a typical Naked Single example.
Analysis Process
Naked Single: R1C9 has only one candidate: 4.
Therefore R1C9 = 4.
Naked Single vs Hidden Single
Naked Single and Hidden Single are the two most basic techniques in Sudoku. Beginners often confuse them. Let's compare to understand the difference:
| Comparison | Naked Single | Hidden Single |
|---|---|---|
| Focus | Single cell | Entire row/column/box |
| Identification | Cell has only 1 candidate | A number has only 1 position in the region |
| Candidate count | Cell has exactly 1 candidate | Cell may have multiple candidates |
| Difficulty | Easier to spot (look at cell) | Requires scanning entire region |
- Naked Single: Look at the cell → What can this cell be?
- Hidden Single: Look at the number → Where can this number go?
Practical Tips
1. Use Candidate Notation
To effectively use Naked Singles, the most important thing is to correctly mark candidates. When you note all possible numbers for each empty cell, Naked Singles will automatically appear—cells with only one candidate are your answers.
2. Update Candidates Promptly
After filling in a number, remember to update candidates in related cells. This may create new Naked Singles:
- Remove that number from other cells in the same row
- Remove that number from other cells in the same column
- Remove that number from other cells in the same box
3. Combine Both Techniques
In practice, Naked Singles and Hidden Singles are used alternately. After placing a number, both situations may occur, so use them flexibly.
Summary
Key points for Naked Single:
- Identification: Cell has only one remaining candidate
- Elimination sources: Numbers in the same row, column, and box
- Difficulty level: Most basic technique; simple Sudokus are mainly solved with these two methods
- When to use: After marking candidates, scan for cells with single candidates
Start a Sudoku game and try using the Naked Single technique! We recommend starting with easy difficulty to master candidate notation first.