An advanced technique that exploits the single-solution property to eliminate candidates that would create deadly patterns.

Unique Rectangle exploits the fact that a valid Sudoku has exactly one solution. When four cells forming a rectangle across 2 blocks could create a deadly pattern (allowing digit swaps without changing validity), candidates that would complete that pattern are eliminated.

Deadly Patterns

A deadly pattern occurs when two digits could be swapped between four rectangular cells without violating any constraint. This would create two valid solutions. Since proper Sudoku puzzles have exactly one solution, any candidate arrangement leading to a deadly pattern must be incorrect.

Prerequisites

This technique is only valid when the puzzle guarantees a unique solution. Puzzles from reliable sources are safe. Use as a last resort after all other logical techniques have been exhausted.