Naked Pair
Naked Pair · Locked Pair
Two cells in a unit have the same two candidates only, so those digits are removed from the rest of the unit.
Rule
Two cells in a unit hold only {x, y} → remove x and y from the other cells of the unit.
Find two cells with the same pair
In row 7, the two cells r7c3 and r7c6 both have exactly the two candidates 1 and 7.
The idea
If two cells in a row can only be 3 or 7, one of them is 3 and the other is 7. We don't know which is which, but the row's 3 and 7 are certainly reserved for those two cells.
So no other cell in the row can be 3 or 7. If the two cells also share a box, you can eliminate in the whole box as well.
How to spot it
- Find cells with exactly two candidates, then compare them with cells in the same row, column and box.
- The two cells must have exactly the same two candidates: {3, 7} and {3, 7}, not {3, 7} and {3, 8}.
Common mistakes
- Two cells that are merely close but not in the same unit are not a naked pair.
- Missing candidates create fake pairs. Write in all candidates before looking for pairs.
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Related techniques
References
The examples on this page are generated from random puzzles, not taken from these sources.