
XYZ Wings 

An XYZWing is like an XYWing in that it is a group of three cells, one sharing a unit with the other two. However, the first cell in an XYZWing has three candidates while the other two, called the wings, have two. Each of the wings must share one candidate with the first cell, (that's part of sharing a unit) but of different values. If the second candidates in the wings are both the same, and are also the same as the extra candidate in the first cell, then if any fourth cell shares that candidate and a unit with all three, that candidate can be eliminated.
Consider the example on this page. The highlighted cell with 1, 3 and 7 as candidates is the first cell, and the other two are the wings. One wing shares the 1 with the first cell, and the other shares the 3. That leaves a 7 in all three cells, which all three share a unit with the 7 highlighted in green. Whether the first cell is a 1, 3 or 7, any way we go, the 7 in green can be eliminated.
Sudoku Snake gives XYZWings a skill value of 180.






