Sudoku Tutorial
XY-wing
An XY-wing involves
• Three candidates
• Four cells (one base, two wings, and a target cell)   (in some cases there can be more than one target cell)
Three of these cells must be bivalue cells  (except for the target cell).

One cell is the BASE or PIVOT (shown in blue here).  The next two bivalue cells must each be able to see the base and are called PINCERS or wings (the yellow cells).  The final cell is the target and must be visible to both pincers.
 The base (blue cell)  contains two numbers, X & Y.   (6 and 7 in this case) The two pincers or wings must have a common candidate Z (9 in this case).  Also one of the pincers must have an X , the other pincer must have a Y.  So pincers are the yellow cells with values  69 and 79 (XZ & YZ). Any cell that is visible to both wings or pincers is a target cell.  The target cell can contain any number of candidates.  If the target cell contains a Z (9 in this example), the 9 can be eliminated .  (The target cell in this example happens to be a bivalue cell but it can contain any number of candidates). If you look again at the blue square, whichever value it has, one of the yellow pincers must be a 9.  The target cell is visible to both pincer cells and therefore cannot be a 9. Another way to say this is that the 9's in the yellow cells are now strongly linked due to their relationship with the base. Not all XY-wings are rectangular.  Click here for an example (an XY-wing is also an XY-chain.  Check this out)

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