XWing

In the puzzle below, we have an example of the XWing technique. The yellow cells indicate the XWing.
Cells A and B share the same row. Since these are the only cells which can contain a 3, either cell A or cell B
must be a 3. Cells C and D also share the same row. These cells are the only ones which can contain a 3. So either cell
C or D must contain a 3. Because cell A and C and cell B and D share the same column, we can use this information to
exclude certain candidate values.
When cell A contains a 3, cells B and C cannot and cell D must be a 3.
When cell B contains a 3, cells A and D cannot and cell C must be a 3.
We can simplify this by saying that when cell A is a 3, cell D must be the other 3. The same goes for
cell B and C. We do not know which is a 3, but we do know that other cells that share the same columns
cannot be a 3. In the example below, we can exclude the candidate value 3 from the blue cells.


