Decabit Software
Block Line

If a value can exist in only one row or column within a block, then the value cannot lie anywhere else in that row or column outside of that block.

Consider the sudoku below.

1
8
4
2
8
5
8

In the center block, the candidate 8 only exists in r4c5 and r5c5. Regardless of which cell the value 8 is in, we know there cannot be another 8 that exists in column 5. So we can remove the candidate 8 from the cells in column 5 that are outside the center block.
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Last Updated: 8/5/2017 12:27:29 PM CST