How it works
XY-wing is the first true chain technique. It tracks consequences across three cells linked by a shared digit Z.
The pivot cell must be bivalue with candidates X and Y. Each wing must also be bivalue. One wing must contain X and Z, and must see the pivot. The other wing must contain Y and Z, and also see the pivot. Wings do not need to see each other.
If the pivot takes X, the X-wing must take Z. If the pivot takes Y, the Y-wing must take Z. Either way Z is placed in one of the two wings. Any cell that sees both wings cannot be Z, no matter how the pivot resolves.
When to look for it
After X-wing fails. Scan for three-cell bivalue chains where two ends share a common digit (Z).
Tips for spotting the pattern
- Find bivalue cells first; chains build from them.
- The pivot must see both wings, but the wings do not need to see each other.
- Z is the digit you eliminate, never X or Y.
Common mistakes
- Wings with three candidates. XY-wing requires every cell to be exactly bivalue.
- Eliminating X or Y. The eliminations always target Z.
- Forgetting the visibility condition. The pivot must see both wings.