Sudoku Tutorial
X-wing explained:

The 4's in row 2 are strongly linked.  One must exist, the other cannot.
Same with the 4's in  row 8.

The 4's in row 2 and row 8 must be diagonally opposed.  (A 4 in row 2 will
eliminate the 4 in row 8)

These are the only two possibilities.  Either way, candidate 4 is now
committed in columns 3 and 6.   Any 4's in the other cells of those columns
can be eliminated

XwingSmall2.jpg   XwingSmall1.jpg

Another way to say this is that the 4's within the columns become strongly linked.
In column 6, there are five 4's.    Any pair of those 4's is weakly linked.  That is
until you find the X-wing.   The X-wing creates a strong link between the 4's in
r2c6 and r8c6 (the yellow squares in column 6).  This strong link means that
you can now eliminate the 4's in the other cells of that column.

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