Intermediate
Hidden quad
Four digits share a four-cell pool between them.
Hidden quad is the digit-perspective mirror of the naked variant. Four digits in a unit collapse onto the same set of four cells. Those cells still wear their fuller candidate lists, but every digit outside the quartet is bogus and can be wiped.
It's the rarest member of the subset family. Most puzzles will never need one.
When the move applies
A pattern reserved for tough puzzles where the easier moves have run out and the candidate grid is still dense.
The procedure
- For a unit, list each digit's remaining candidate cells.
- Find four digits whose combined candidate cells are exactly the same four cells.
- Strip every other candidate from those four cells.