Block & Block Interaction

This is where you can remove a number as a possibility in certain squares by examining how a block interacts with another block. Consider the following board:

In the first block, a 7 can only go in either of the two highlighted cells, and in the second block, a 7 can only go in either of the two highlighted squares. Because all four of these squares are in the same two rows, we can deduce that a 7 must be in row two in either block one or two and another 7 must be in row three in the other block. Therefore, since we know a 7 must be in both rows two and three, we can eliminate 7 as a possibility from rows two and three in block three.

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