A Unique Rectangle is a different kind of deduction from the others on this page. Instead of pure elimination logic, it leans on one fact about how Sudoku puzzles are built: a properly constructed puzzle has exactly one solution.
If four cells form a rectangle across exactly two rows, two columns, and two boxes, and all four can only hold the same two digits, then swapping which cell gets which digit would produce a second, equally valid solution to the whole puzzle. Since a real puzzle can't have two solutions, that configuration is never actually the answer — which lets a solver rule out the move that would create it.
These four cells sit in two rows, two columns and two boxes, and involve only 3 and 9.
A valid puzzle has only one solution, so it can never end with a rectangle where the two digits could be swapped. Solvers use that to rule out moves that would create one.
KaizenDoku spots every one of these patterns automatically after each game, and shows you exactly where you used them on your own board.
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