Hidden Pair
A Hidden Pair shows up when two digits can only go in the same two cells inside one house — a row, column, or 3×3 box. Those two cells have to hold both digits between them, so any other pencil-mark candidate sitting alongside them in those cells is dead weight and can be erased.
What is a Hidden Pair?
A Hidden Pair is the mirror image of a Naked Pair. With a Naked Pair, two cells each carry exactly the same two candidates — the pair is right there in the notes. With a Hidden Pair, the pair is buried: the two cells that share the locked digits also have other candidates, so you have to look for the digits that only appear in those two cells, not the cells that only have two candidates. Once you find it, the payoff is the same kind of cleanup — but this time you're stripping extra notes from inside the pair cells, not from the rest of the house.
How to spot it
Pick a house and, for each digit, count how many cells can still hold it. When two different digits each have exactly two candidate cells left, and those two cells are the same pair, you've found a Hidden Pair. It's less obvious than a Naked Pair because the cells carry extra notes that camouflage the pattern, so scanning by digit rather than by cell is the reliable way in. Hidden Pairs turn up regularly in mid-difficulty puzzles, often after the singles and intersections have run their course.
The logic
Every house has to place each digit 1–9 exactly once. If two digits can only go in the same two cells in a house, one of those cells gets the first digit and the other gets the second — there's no third option, because neither digit appears anywhere else in the house. That means both cells are fully spoken for: they'll contain exactly those two digits, in some order. Any other candidate note inside them is impossible, so it can be crossed off. The same reasoning with three digits in three cells gives a Hidden Triplet, and with four, a Hidden Quad.