
XY Wings 

An XYWing is a group of three cells, one sharing a unit with the other two, each having only 2 candidates. The two cells that share a unit with the first are called the Wings. 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 both share a unit with a common candidate in a fourth cell, that candidate can be eliminated.
Consider the example on this page. C8 is the first cell, and the other two are the wings. A9 shares the 4 unit with the first cell, and C5 shares the 5. The remaining values are both 1's. If the first cell is a 4, A9 is a 1. If the first cell is a 5, C5 is a 1. Either way, they both cause the elimination of the 1 highlighted in green.
Sudoku Snake gives XYWings a skill value of 120.






