A hidden pair occurs when two digits can appear only in the same two cells of one row, column or box. Those cells must contain the pair, even if their notes currently include other digits.
Count digit locations, not just cell notes
Suppose a row has candidates {2,7,9} in one cell and {2,4,7} in another. If 2 and 7 appear nowhere else in that row's candidates, remove 9 from the first and 4 from the second. You still do not know which cell gets 2. The conclusion depends on complete, correct candidate lists across the entire unit.
Try the steps
- Choose one row, column or box.
- Find two digits whose possible locations are exactly the same two cells.
- Remove other candidates from those cells and recheck the affected units.
How is this different from a naked pair?
A naked pair begins with two cells already containing only the same two candidates. A hidden pair begins with the two digits' restricted locations, so extra candidates inside those cells can be removed.