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Unique Rectangle Type 4

When one base candidate is locked into the roof cells.

Explanation

Type 4 looks at the base pair instead of the extra candidates. If one base candidate can only appear in the two roof cells inside their shared house, that base candidate must occupy one roof cell, so the other base candidate can be removed from both roof cells.

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

Practice tip

Find the two roof cells first, then scan their shared row, column, or box for where each base candidate can still appear.

Example steps

  1. Find a unique rectangle with two floor cells holding the base pair.
  2. Check the shared house of the two roof cells.
  3. If one base digit appears only in the roof cells, remove the other base digit from both roofs.

Try this technique on a real puzzle.

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