Unique Rectangle
Prerequisites: Naked Pair
Overview
A Unique Rectangle (Type 1) exploits the fact that a valid sudoku puzzle has exactly one solution. If four cells forming a rectangle across two boxes would create an ambiguous pair, extra candidates in the fourth corner must be the solution.
How It Works
Find four cells forming a rectangle (2 rows x 2 columns) that spans exactly two boxes. If three corners are bivalue with the same pair (A,B), the fourth corner must not also be (A,B) -- otherwise the puzzle would have two solutions (you could swap A and B in all four cells). Therefore, digits A and B can be eliminated from the fourth corner.
When To Use It
Look for Unique Rectangles when you see three bivalue cells with the same candidate pair forming three corners of a rectangle across two boxes. Check the fourth corner for extra candidates.
How To Detect It
Find pairs of bivalue cells with identical candidates. Check if three such cells form three corners of a rectangle spanning exactly two boxes. If the fourth corner contains the same pair plus extras, eliminate the pair digits from it.
Visual Example
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Uniqueness as a Solving Tool
A Unique Rectangle (Type 1) exploits a fundamental property of valid sudoku: the puzzle has exactly one solution. If a pattern would allow two valid solutions, the puzzle would be invalid — so we can rule out that pattern.
Practice
A Unique Rectangle is hiding here. Use the uniqueness rule to eliminate, then fill the Naked Single.