Unique Rectangle
Four cells in a 2-row × 2-column × 2-box rectangle sharing the same two candidates form a deadly pattern. Eliminate anything that would lock in that pattern.
Advanced sudoku technique
What it is
A Unique Rectangle (UR) is a pattern that would create TWO valid sudoku solutions if it appeared in the final grid — and since every legitimate puzzle has exactly one solution, the pattern can't occur. The setup: four cells forming a rectangle spanning two rows × two columns × two 3×3 boxes (so two cells in each box), where all four cells share the same two candidates {X, Y}. If those four cells ended up as {X, Y, Y, X} or {Y, X, X, Y}, both arrangements would solve the puzzle equally well — a 'deadly pattern' with two solutions. So the pattern must be broken: at least one of the four cells must hold a candidate OTHER than X or Y. That third candidate forces a unique placement and lets you eliminate X and Y from the relevant cells. UR has several sub-types (Type 1 is the most common: three cells have only {X, Y} and one cell has {X, Y, Z}).
Want the full theory?
The Unique Rectangle guide on Sudoku247Wiki walks through the logic step by step, with worked examples, diagrams and FAQs.
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