Hidden single
EasySometimes a digit fits in only one cell of a row, a column or a box: every other empty cell there already sees that digit, in its own row, column or box. That cell is a hidden single, even if other digits could go in it too.
How to spot it
- Pick a digit and a box. Boxes are the quickest to scan.
- Rule out every empty cell of the box that already sees the digit.
- If one cell is left, the digit goes there.
Why it works
The digit must appear once in the box (or row, or column). If every other free cell already sees it, it has nowhere else to go.
Example
7
1
5
6
2
3
9
3
8
1
7
5
7
8
1
8
2
3
9
5
6
4
7
9
5
7
6
4
8
3
1
2
3
6
4
1
2
7
2
3
5
1
8
4
3
9
7
5
2
Look at box 1: an 8 fits in only one of its cells.
Common mistakes
- Forgetting that a cell sees its own row and column too, not just its box.
- Stopping at one box: the same digit is often a hidden single in several places.