Naked single
MediumSometimes a cell can hold only one digit: the eight others are already in its row, its column or its box. That digit is a naked single.
How to spot it
- Pick a cell with many digits around it.
- Go through the digits already in its row, its column and its box.
- If eight of the nine are there, the last one goes in the cell.
Why it works
A cell must hold one of the nine digits. If eight of them are already in its row, column or box, only the ninth is left.
Example
6
5
7
4
9
8
1
7
1
5
1
4
3
9
5
7
6
4
2
8
6
4
1
5
3
8
2
9
7
7
1
9
5
6
4
1
6
4
8
2
4
1
6
5
3
2
8
6
9
Look at row 9: one of its cells has only one digit left that fits.
Common mistakes
- Counting a digit twice when it is in both the row and the box, and missing the one really left.
- Confusing it with a hidden single: here you count digits for one cell, there cells for one digit.