Techniques · Expert
Quads
A quad is the subset pattern at four. A naked quad is four cells in one house whose combined candidates are exactly four digits. Those digits leave the rest of the house. A hidden quad is four digits that fit in only four cells of a house, so every other candidate on those cells is removed.
Quads are easy to miss because four cells is a lot of the house, and the elimination is often small. They are also rare in newspaper puzzles. This site keeps them for Expert, where the puzzle is allowed to ask for one.
When it shows up
Quads are an Expert technique, with X-wings, swordfish, and XY-wings. A puzzle whose hardest step is a quad is Expert, not Hard.
Across all 1,483 NYT puzzles in the archive, the solver never needs a quad. Hard stops at triples.
See it on a real puzzle
The marks are the candidates left when this is the next step in the solve. Step through it: the pattern, what leaves, and what that forces.
See the pattern
Look at these four cells.