XY-Wing

Index
XY-Wing
The XY-Wing is based on the same principles as the Naked Pair technique. Instead of using two candidate values in two cells, this technique uses three candidate values in three cells.

In the example below a XY-Wing is explained. Cells A, B and C mark the XY-Wing pattern. What we are going to prove is that cell A or cell C must contain the value 7. These to cells are linked together by cell B.

When cell A has the value 6, cell B has the value 1 and therefore cell C must have the value 7.
When cell C has the value 1, cell B has the value 6 and therefore cell A must have the value 7.

When we simplify this we can say that when A has the value 6, C must have the value 7. Because of this, we can say that the blue cell can not be a 7. This candidate value can be removed.
XY-Wing