Unique Rectangle
Exploits the single-solution guarantee: four cells forming a rectangle across two boxes with the same pair would allow two solutions, so the deadly pattern must be broken.
Four cells form a rectangle across two rows and two boxes. Three show only {3,8}.
How to spot it
- 1Spot four rectangle corners across two rows, two columns, two boxes.
- 2If three share a pair {X,Y} and the fourth adds extras, uniqueness is at risk.
- 3Eliminate {X,Y} from the fourth cell to break the pattern.
When to use
Only on puzzles guaranteed to have a unique solution — which all proper ones are.
When not to
Never on hand-made or variant puzzles that might not be unique.
Common mistake
Applying it when the four cells span more than two boxes — then it isn't a deadly pattern.
Worked Example
A unique rectangle exploits a guarantee every proper Sudoku makes: exactly one solution exists. If four cells form a rectangle across exactly two rows, two columns, and two boxes, and three of those corners show only the same two candidates {X,Y}, then a fourth corner showing just {X,Y} too would create a "deadly pattern" — the puzzle could swap X and Y across all four corners and still be valid, meaning two solutions instead of one.
Since that's impossible in a proper puzzle, the fourth corner must carry some other candidate beyond {X,Y}, and those two candidates can be eliminated from it to force a genuine third digit. This technique only applies to puzzles guaranteed to have a single solution — never to hand-made or variant grids where that guarantee might not hold.