Hidden pairs are the trickier counterpart to naked pairs. Instead of two cells that each contain only two candidates, you find two digits that together only appear in two cells within a unit — even though those cells may have many other candidates cluttering them.
In a naked pair, the two-candidate cells jump out immediately. In a hidden pair, the pair is buried. The two cells have other candidates too. You only spot the pair by noticing that two specific digits have no other home in that unit.
Look at digits 4 and 9. Where can they go in row 3? Only cells A, D, E, I for 4 — but check further: 9 only appears in A, E, I. And 4 appears in A, D, E. The intersection where both can appear is A and E. Wait, let me recalculate...
The simpler reading: scan each digit and note which cells in the row contain it. If two digits each appear in exactly the same two cells and no others, those two cells form a hidden pair.
After the elimination, cells C and G contain only {3, 8} — which is now also a naked pair, enabling further deductions.
This requires maintaining or scanning candidate lists, which is why hidden pairs rarely appear in beginner solving. They're a mid-to-advanced technique for when basic scanning has stalled.
The same pattern extends to three digits in three cells. Three digits that collectively only appear in three cells within a unit form a hidden triple. Eliminate all other candidates from those three cells. Hidden triples are harder to spot but appear in expert puzzles regularly.