Start with the idea
Start with one empty square. In this example, the center square can see 1, 2, 3, 4, 6, 7, 8, and 9 in its row, column, or 3x3 box. The only number left is 5, so 5 is the correct number for that square.
Example
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
Look here firstStart with the center square. Its row, column, and 3x3 box already rule out every number except 5.
This is a square-solving technique: the correct number for the center square is 5.
A naked single is ready to place because only one possible number remains in that square.
- Start with one empty square.
- Check its row, column, and 3x3 box.
- Place the only number that is still possible.
square to fillfilled squares to check
When to look for it
Use it whenever Pencil Mode or row/column/box scanning leaves one possible number in a square.
How to use it
- Pick an empty square with very few possible numbers.
- Look at the filled numbers in its row, column, and 3x3 box.
- If every number but one is already ruled out, place the number that is left.
Common mistakes
- A naked single is about one square, not one whole row or box.
- If two possible numbers remain, keep scanning or use another technique first.