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Blocked Blocks

The Blocked Blocks method is one of the easiest higher methods. Using higher methods is worthwhile only when the two basic methods provide no further progress.

In the Blocked Blocks method you look at the fields of an intersection that occurs between a block and a line or a block and a column.
Choose one perspective: Look at the intersection either from the block perspective, or from the row/column perspective.

Assume you look at the intersection between a block and a line from the block perspective. Then the following must be true: If a candidate in the block appears only in the intersecting fields, it can be deleted from all fields in the line that do not belong to the intersection.

It also works in the other direction - from the second possible perspective.
Assume you look at the intersection between a block and a line from the line perspective. Then the following must be true: If a candidate in the line appears only in the intersecting fields, it can be deleted from all fields in the block that do not belong to the intersection.

Explanation (for the second perspective): If you entered the candidate in one of the block fields that do not belong to the intersection , then the candidate would no longer appear in the line. It would be "lost" in the line.

Both conclusions can of course be applied in the same way to the intersection between a block and a column.
This, again, is a case in which the theory sounds terribly complicated. In fact, it's often very easy to see in practice.

The intersection is marked in green. Second angle.

In the line, the 7 can only be within the intersection. Consequently it can be deleted in the yellow fields, because if it were there, they would be "lost" in the line.

This example of Blocked Blocks is relatively hard to see!

Sudoku example blocked Blocks

This example of Blocked Blocks is much easier to see.

In the intersection, marked in green, the candidates 2 and 5 form a Hidden Twin (from the block perspective).

Both candidates can be used wonderfully for Blocked Blocks, because you can eliminate candidates in the yellow fields.

Sudoku example blocked Blocks
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