A hidden pair hides in plain sight: two digits that can only be placed in the same two cells of a unit. Those cells may show four or five pencil marks each, which is exactly why players miss it — but since the two digits must occupy the two cells between them, every other candidate written there is dead weight and can be erased.
Digits 4 and 7 fit only r2c3 and r2c8 in row 2 — all other candidates in those two cells are erased.
Row 2 still needs 4 and 7, and when you check where each can go, both digits fit only r2c3 and r2c8 — every other cell of the row is blocked by a 4 or 7 in its column or box. Those two cells may currently show marks like 3-4-6-7 and 4-5-7-9, but the pair claims them: strip everything except 4 and 7, and the row usually unravels from there.
Confirm both digits are genuinely limited to the same two cells; a third possible home for either digit breaks the pair. This is where sloppy pencil marks hurt most — a missing mark fakes a pair that isn't there.
These grids come from rated games on this site — our solver confirmed that hidden pair was part of the shortest solving path for each one.
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