Techniques · Hard
Naked pair
A naked pair is two cells in the same row, column, or box whose only candidates are the same two digits. Those two cells will be those two digits, in one order or the other. Either way, both digits are used up, so they come out of every other cell in the house.
The cells have to share a house. Two cells with {2, 4} that don't see each other are not a pair, and they eliminate nothing.
When it shows up
Hard puzzles require a pair or a triple, and at least four steps that are not singles. A naked pair is enough to put a puzzle in Hard.
A naked pair is the hardest step on 51% of the 494 NYT Hard puzzles. Nothing in the archive goes beyond 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 two cells.