Sudoku Tutorial
Uniqueness Techniques
deadly.jpg There are several solving techniques which involve avoiding a deadly rectangle situation.   Deadly rectangles can only exist in invalid puzzles (they have more than one solution).  

The four cells shown on the left form  a deadly rectangle.   There is no way to remove a candidate so there are two solutions  to this invalid puzzle.

IMPORTANT:  The four cells must reside in only two boxes.  The cells cannot reside in four different boxes.
(see this example)

There is a large number of uniqueness tests.  Only a handful of  them are shown on this tutorial.  

(Is this a valid solving technique?)


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