Sudoku Tutorial
Strong Links   (A or B is true, the other is false)                      (not necessarily, read this)

Strong link:  When two numbers are connected by a strong link, one must exist, the other cannot.   The two numbers
in question can be related by residing in the same house but they can also be linked by a solving technique.  In that case, the
numbers can reside in unrelated houses (see the XY-wing for an example).

When a candidate is present in any house exactly two times, the candidate is strongly linked.
One must exist, the other cannot.

stronglink.jpg

In this row, the 6's have a strong link.  One cell must be a 6, the other cannot.

There is one other type of strong link.  That is where two different
candidates reside in the same cell.  (look at the yellow cell)

stronglink2.jpg

In this case, the 6 and 8 are strongly linked in a cell.  One must exist, the other cannot.  This type of strong
link is useful when performing chains.
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