Hidden quad
ExpertWhen four digits can only go in the same four cells of a row, a column or a box, those cells hold those four digits. Every other candidate in them can go. It is the rarest of the subsets.
How to spot it
- Pick a unit and note where each missing digit can go.
- Find four digits whose places, together, are only four cells.
- Remove every other candidate from those four cells.
Why it works
The four digits must each go somewhere in the unit and have only four cells between them, so they fill those cells.
Example
6
2
3
1
8
5
2
3
5
4
2
1
6
9
3
2
1
6
2
1
4
8
2
1
4
6
3
1
4
2
3
7
9
6
4
1
Look at where 1, 2, 3, and 4 can go in column 5.
Common mistakes
- Looking for it before simpler steps: pairs and triples come first and are far more common.
- Removing the four digits from other cells: they weren’t there to begin with.