Hidden pair

Hidden pair is the mirror image of naked pair. Instead of two cells reducing to two candidates, two digits reduce to two cells. If digits 4 and 9 can each only go in cells A and B within a unit, then no other digit can occupy A or B — even if A and B currently have more candidates listed.

Hidden pairs are harder to spot because the cells still 'look' busy. You find them by scanning digits, not cells.

When to look for it

For each unit, take inventory: count how many cells each unplaced digit could occupy. When two digits both have exactly the same two-cell pool, you've found a hidden pair.

How to apply it

1. Within a unit, list every cell each digit could still go into.
2. Find two digits that share exactly the same two cells.
3. In those two cells, erase every candidate that isn't one of those two digits.

Example