Sudoku Tutorial
Unique Side
unique_side.jpg The four yellow cells have the potential to form a deadly rectangle.

Look at the yellow cells in column 4.

The trick here is to look for a strongly linked 4 or 8 in column 4.  

The rule is this:  "Of candidates X and Y, if candidate X is strongly linked, candidate Y can be eliminated"

In this case, the 8's in column 4 are not strongly linked (there are four of them).  However the 4's are strongly linked (there are only two of them).  So if either of the yellow cells in column 4 is an 8, the other yellow cell is forced to become a 4.  This would complete the deadly rectangle.  So both 8's can be eliminated.

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