Tier 2 · first asked for by Moderate
Hidden Pair
Two digits confined to the same two cells push every other candidate out of them.
Take a unit and look for two digits that can each only go in the same two cells. Between them they will fill both, so nothing else can — every other candidate in those two cells can be rubbed out.
This is the dual of a naked pair, and much harder to see, because you are looking for something that is not there: the two cells may be cluttered with five or six candidates each, and the pair only becomes visible once you count where each digit in the unit can actually go.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. In column 7, 1/3 can only live in R4C7, R6C7, so those cells hold nothing else.
Builds on Naked Pair.