XY-Wing
A cell with two candidates links to two other two-candidate cells that share one candidate each with it and one with each other — the shared value can't be in any cell both of those see.
How it works
- 1The top-left pivot has {1,2}. Its row wing has {1,3}, and its column wing has {2,3}.
- 2If the pivot is 1, the row wing must be 3. If the pivot is 2, the column wing must be 3. Either way, one of the two wings is 3.
- 3The cell at row 3, column 3 sees both wings — one through its column and one through its row — so it cannot be 3.
- Candidates this technique rules out
- Cells read together — The connecting line shows which cells combine to make the deduction.
- Conclusion — The coloured candidate, or the final arrow when it lands outside the pattern, shows the result.