Unique Rectangle Type 1
When one corner must break a deadly rectangle.
Explanation
A Unique Rectangle uses the assumption that a valid Sudoku has one solution. If four cells across two rows, two columns, and two boxes would become the same two-candidate rectangle, the puzzle would allow the pair to be swapped. In Type 1, one corner has extra candidates, so the base pair can be removed from that corner.
Practice tip
Check that the four cells occupy exactly two boxes. If the rectangle spans four boxes, this technique does not apply.
Example steps
- Find four cells across two rows and two columns.
- Confirm three cells contain only the same candidate pair.
- Remove that pair from the fourth cell with extra candidates.