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Easy

Naked Single

A Naked Single is the most basic solving step in Sudoku, and usually the first one any solver learns without even naming it: a cell where only one digit is left once you rule out everything already used in its row, column, and box.

Why it works

Every row, column, and 3×3 box must contain each digit 1–9 exactly once. If a cell's row, column, and box between them already account for eight of the nine digits, only one digit can possibly go in that cell — it's forced, not guessed.

Worked example

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 1 of 3

Find the cell

R5C5 is empty. Check which digits it can still hold.

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 2 of 3

Rule out digits

The highlighted cells share a row, column or box with R5C5. Between them they already use every digit except 5.

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
5
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 3 of 3

Place it

Only 5 is left, so R5C5 must be 5.

Tips

KaizenDoku spots every one of these patterns automatically after each game, and shows you exactly where you used them on your own board.

Download on the App Store

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