Naked Pairs
If two Cells in a region (Row, Column or Square) contain exactly the same two candidates, then one of these candidates is the solution for one of these Cells and the other candidate is the solution for the other Cell.
Hence none of these two candidates can be the solution in any other Cell of that region; these two candidates can be deleted from the other Cells of that region.
You can practice this strategy by installing the free SudokuCoach application on your Android™ device.
