Naked pair
HardTwo cells of the same row, column or box that can only hold the same two digits share them: one cell gets each. So neither digit can go anywhere else in that row, column or box.
How to spot it
- With notes, look for two cells of a unit with the same two candidates, and nothing else.
- Those two digits belong to the pair.
- Remove them from the other cells of the row, column or box.
Why it works
Two cells need two different digits and only have two to share, so they take both. No other cell of the unit can have either.
Example
4
3
7
1
5
3
5
6
1
4
1
5
3
6
3
4
5
2
8
2
5
9
4
6
3
5
3
4
8
6
2
1
2
5
6
3
4
5
3
9
3
2
8
5
Look at row 9.
Common mistakes
- Taking two cells that only share their two candidates in part: both must have exactly the same pair.
- Removing the pair from cells outside the unit the two cells share.