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 height can't be in any cell both of those see.
How it works
- 1Three cells with two candidates each. The pivot in the top-left corner is {1, 2}; it shares its row with a {1, 3} cell and its column with a {2, 3} cell.
- 2Suppose the pivot is 1. Then the {1, 3} cell in its row can't be 1, so it's 3.
- 3Suppose instead the pivot is 2. Then the {2, 3} cell in its column can't be 2, so IT is 3.
- 4Either way one of the two wings is a 3 — we never find out which, and we don't need to.
- 5Any cell that sees BOTH wings therefore can't be 3. The opposite corner sees the first down its column and the second along its row. Strike its 3.
- Height ruled out
- Relationship — The arrow shows which cells support the deduction.
- Conclusion — The final arrow lands beside the proved or ruled-out height.