Uses the principle that a valid Sudoku cannot have a deadly pattern (multiple solutions) to eliminate candidates.
If four cells forming a rectangle across two rows, two columns, and two boxes all share only the same two candidates, the puzzle would have multiple solutions (a "deadly pattern") — which is impossible in a valid puzzle. Therefore, the candidate that breaks the pattern must be the solution.
Type 1: Three corners {A,B}, one corner {A,B,+extra} → Eliminate A,B from the extra corner. Type 2: Two corners {A,B}, two corners {A,B,C} → Eliminate C from cells seeing both type-2 corners. Types 3–6: More complex variations.
37 | 37 | 2 | 4 | 9 | 6 | 5 | 1 | 8 |
| 1 | 4 | 5 | 2 | 3 | 8 | 67 | 67 | 9 |
| 6 | 8 | 9 | 1 | 5 | 7 | 2 | 3 | 4 |
| 2 | 1 | 6 | 7 | 4 | 3 | 9 | 8 | 5 |
| 8 | 37 | 347 | 5 | 6 | 9 | 147 | 2 | 17 |
| 9 | 5 | 47 | 8 | 1 | 2 | 3 | 47 | 6 |
34 | 6 | 13 | 9 | 7 | 14 | 8 | 5 | 2 |
| 5 | 2 | 17 | 6 | 8 | 14 | 47 | 9 | 3 |
47 | 9 | 8 | 3 | 2 | 5 | 1467 | 467 | 17 |
This 9×9 puzzle is solver-verified to require this technique on its solution path.