Pick a unit — a row, column or box — and write in its pencil marks.
Hidden Pairs
Two digits, two cells — everything else in those cells can go.
A hidden pair is the mirror image of a naked pair. Instead of two cells that hold only two candidates, you find two digits that can only go in the same two cells of a row, column or box. Those two cells must hold those two digits, so any other candidates in them can be erased — even though the pair was 'hidden' among other pencil marks.
Worked example
146789
5
2
146789
3
9
1
1
1
9
9
How to apply it
For each digit still missing from the unit, note which cells can take it.
Look for two digits whose possible cells are exactly the same two cells.
Those cells are locked to those two digits: remove every other candidate from them.
The cleaned-up cells now form a naked pair, which may trigger further eliminations.
When to use it
Hidden pairs show up on hard puzzles once singles run dry. They are harder to see than naked pairs because the cells still look crowded — scanning digit by digit through a unit is the reliable way to find them.
Frequently asked questions
What is the difference between a naked pair and a hidden pair?
A naked pair is two cells that contain only the same two candidates. A hidden pair is two digits that can only go in the same two cells, while those cells may still show other candidates. Both lock two digits into two cells.
Are there hidden triples too?
Yes. Three digits confined to the same three cells of a unit form a hidden triple, and the same logic applies. Hidden quads exist but are rare.