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XWing Strategy From sudokuwiki.org, the puzzle solver's site 
The picture on the right shows a classic XWing, this example being based on the number seven. The X is formed from the diagonal correspondence of squares marked A, B, C and D. What's special about them? Well, A and B are a locked pair of 7's. So is C and D. They are locked because they are the only 7's in rows B and F. We know therefore that if A turns out to be a 7 then B cannot be a 7, and vice versa. Likewise if C turns out to be a 7 then D cannot be, and vice versa. What is interesting is the 7's present elsewhere in the fourth and eighth columns. These have been highlighted with green boxes. 
XWing example1: Load Example or : From the Start 
In this second example I've chosen a Sudoku puzzle where an enormous number of candidates can be removed using two XWings. The first is a '2Wing'. The yellow high lighted cells show the XWing formation. Note that the orientation is in the columns this time, as opposed to rows as above. Looking at columns we can see that candidate 2 only occurs twice  in the yellow cells. Which ever way the 2s could be placed (E5/J8 or E8/J5) six other 2s in the rows can be removed  the green highlighted cells. 
XWing example 2: Load Example or : From the Start 
A few steps later the second XWing is found on candidate 3 in the same rows. Whichever way round the 3 can be placed in those rows (E2/J8 or E8/J2) there can be no other 3 in rows E and J except in those yellow cells.  XWing example 3 

