Hidden Pair

Sometimes two digits can only appear in the same two cells of a unit, but those two cells still list other candidate digits alongside them. The pair is 'hidden' inside cells that look busier than they really are.

Since those two digits must occupy exactly those two cells, every other candidate in those two cells can be eliminated — collapsing them down to a naked pair.

Hidden pairs are easy to overlook because the two cells involved don't look related at a glance — they can be scattered across a unit with several other candidates cluttering them. Tracking where each unplaced digit can still go, one digit at a time, is what surfaces them.