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Unique Rectangle Type 2

When two roof cells share the same extra candidate.

Explanation

Type 2 starts with the same deadly rectangle idea: four cells across two rows, two columns, and two boxes could collapse to one interchangeable pair. If the two non-diagonal roof cells both contain the same extra candidate, at least one roof must use that extra candidate, so matching candidates that see both roofs can be removed.

Unique Rectangle Type 2 example Sudoku board 4 6 7 1 5 9 2 5 1 3 9 7 6 7 2 3 8 6 4 5 8 2 5 1 3 9 4 3 7 2 8 3 2 8 4 6 5 7 1 8 3 4 2 4 8 5 3 9 1 3 9 6 7
An example board where Unique Rectangle Type 2 applies

Practice tip

First confirm the two floor cells contain only the base pair. Then look for the shared extra note in both roof cells.

Example steps

  1. Find four cells across two rows and two columns.
  2. Confirm the two floor cells contain the same base pair.
  3. Remove the shared extra candidate from cells that see both roof cells.

Try this technique on a real puzzle.

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