Techniques · Hard
Hidden pair
A hidden pair is two digits that appear in only two cells of a house. Those cells often carry extra candidates, which is why the pair is hidden. The two digits have nowhere else to go, so they own the two cells, and every other candidate on those cells is removed.
After that cleanup, the two cells are an ordinary naked pair. The hidden pair is the step that gets you there.
When it shows up
Hidden pairs are a Hard technique, same grade as a naked pair. The puzzle still has to need a pair or a triple, with at least four non-single steps.
A hidden pair is the hardest step on 18% of NYT Hard.
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 two cells.