Proven Sudoku
Hidden Quad
Also called Hidden Quadruple, Concealed Quad.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Four digits in a group can only go in the same four squares. Those squares are reserved for them, so clear the other digits out.
The exact rule, for stronger players
The rule, stated exactly: For a house H, if four digits are collectively confined to the same four cells of H, all other candidates may be eliminated from those four cells.
What it removes: Remove every candidate outside the four digits from the four cells.
Why it works: As for the hidden triple, extended by one.
Worked examples
These come from real Proven Sudoku boards. Nobody drew them by hand to make the pattern look neat. Each one is checked first, so the board is one you could really meet, the pattern is really there, and every digit it rules out is really wrong.
Learn these first
- Hidden TripleThree digits in a group can only go in the same three squares.
Related subsets
- Hidden Subset (general form)Some numbers in a group have only a few homes left.
- Locked PairA pair that shares two groups at once, usually a row and a box.
- Locked TripleA triple that sits in two groups at once, usually a row and a box.
- Naked PairTwo squares in the same group both hold only the same two digits.
- Naked QuadFour squares in one group share the same four digits between them.
How often does it come up?
Our rough estimate: rare. Its complement, a naked pair, is almost always visible first.