ALS-XY-Wing
Three almost-locked sets chain through a pivot to trap a digit.
Learn the ALS-XY-Wing Sudoku technique: two wing almost-locked sets join through a pivot set by two restricted commons, trapping a shared digit Z in the wings.
How to use it
- Find the pivot and two wings. Take three almost-locked sets: a pivot set linked to each of two wing sets by a restricted common candidate — X to the first wing, Y to the second, with X and Y different.
- Follow the forced chain. If a wing does not take the shared digit Z, the restricted commons march through the pivot and force the other wing to take Z — so Z must appear in one wing or the other.
- Eliminate Z outside the wings. Because Z is trapped in the two wings, remove Z from any cell outside the three sets that sees every Z candidate in both wings.