Intermediate
Hidden pair
Two digits fit in only the same two cells, even when those cells look busy.
A hidden pair flips the naked pair on its head. Where a naked pair finds two cells with the same two candidates, a hidden pair finds two digits with the same two homes. If digits 4 and 9 can each only land in cells A and B inside a unit, then A and B are sealed off — neither one can hold any of the other digits they might currently list.
Hidden pairs are sneakier than naked ones because the cells still wear their full candidate list. Find them by scanning digits across the unit, not by scanning cells.
When the move applies
After a few placements, take inventory of each digit's remaining homes in a tight unit. Two digits with matching two-cell pools is the tell.
The procedure
- In a unit, list every cell each unplaced digit could still occupy.
- Find two digits whose lists are identical and contain exactly two cells.
- In those two cells, erase every candidate that isn't one of those two digits.