Mirrored from Sudopedia, the Free Sudoku Reference Guide
Upto 2 candidates can be eliminated by an XYZ-Wing, because they need to share an intersection with the pivot. The following diagram shows how it works:
.-----------.----------.----------. | * * XYZ | . . . | YZ . . | | . . . | . . . | . . . | | XZ . . | . . . | . . . | :-----------+----------+----------:
The pivot has candidates XYZ. The implications of each option are:
Under all circumstances, the starred cells will lose their candidates for digit Z.
The XYZ-Wing can be replicated by an ALS-XZ move.
Consider set r1c37. 2 cells, digits XYZ. Consider set r3c1. 1 cell, digits XZ. X is common restricted. It cannot appear in both sets at the same time. One of these sets will be locked for the remaining digits. r1c12 can see all candidates for digit Z in both sets. Since one of these sets will be locked with digit Z, we can eliminate digit Z from r1c12.
This is the Eureka notation for the ALS alternative:
(Z)r1c12-(Z=YX)r1c37-(X=Z)r3c1-(Z)r1c12 => r1c12<>Z
XYZ-Wings can also be replicated by Aligned Pair Exclusion, by pairing one of the target cells with the XYZ cell.