Unique Rectangle
Extreme
Used by Smart Hint
Uses the fact that a proper Sudoku has exactly one solution to rule out candidate patterns that would create a second, deadly duplicate.
- Cells that establish the pattern
- Where the conclusion lands
- Candidate ruled out
How it works
- B1, E1, and B4 all have exactly the same two candidates, {2, 5} — three corners of a rectangle across two rows, two columns, and two boxes.
- If E4 were also just {2, 5}, the puzzle would have two valid solutions, since 2 and 5 could swap around the rectangle — which a real Sudoku never allows.
- So E4 can't be purely {2, 5} — eliminate both, leaving only its extra candidate, 9.