A hidden quad occurs when four digits are confined to the same four cells in a house.
Understanding the Concept
Four specific digits can only appear in four specific cells within a house.
These cells usually contain additional candidates, hiding the quad.
Once found, remove all other candidates from these four cells.
Hidden quads are extremely rare and very difficult to spot.
Often it's easier to find the complementary naked quint (5 cells).
Most puzzles can be solved without finding hidden quads.
Examples
Hidden Quad in Box
In this box, digits 2, 4, 5, and 8 can only appear in four specific cells. These cells have other candidates too, but we can eliminate everything except {2,4,5,8} from them.
123456789
7
9
6
6
7
9
123456789
3
6
7
7
3
6
3
7
4
6
8
2
8
7
3
9
2
6
3
9
7
Pattern / InvolvedEliminated
Pro Tips
- •Hidden quads are extremely rare in practice
- •If you think you need one, re-check for easier techniques first
- •Consider looking for the complementary naked quint instead
- •More of theoretical interest than everyday practical use
