Hidden Quad
Four digits restricted to the same four cells, letting you clear everything else from them.
What it is
A hidden quad occurs when four digits can only be placed in the same four cells of a unit. Those cells must hold exactly those digits, so all other candidates in them disappear. It is the largest of the hidden subsets and the hardest to see, since the cells involved are typically crowded with pencil marks.
How to spot it
List, for every missing digit in a unit, the cells where it could go. Then look for four digits whose combined cell sets cover exactly four cells. This is genuinely laborious, so it is a last resort rather than a routine check.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 1/5, 1/6, 1/9, 1/2
- Digits
- 1, 2, 5, 7
- Eliminations
- 1/5, 1/6, 1/9, 1/2
Press a marked square to point it out on the board above.
Why it matters
Hidden quads are rare enough that many solvers never deliberately look for one. They matter mainly in expert puzzles engineered to require them, where the payoff is a large simplification of a stubborn unit.
Frequently asked questions
How common are hidden quads?
Very rare in normal play. Most puzzles that appear to need one can be solved another way, so treat it as a curiosity unless you are tackling deliberately extreme grids.
Is there an easier equivalent?
Often yes. If a hidden quad exists in a unit with only five empty cells, there is usually a naked single or naked pair that achieves the same thing with far less work.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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