What is a unique rectangle in Sudoku?

What is a unique rectangle in Sudoku?

Somewhat of a controversial subject in the Sudoku world, a unique rectangle (UR) is a situation in which a puzzle may have two different solutions. Hodoku notes that, “A unique rectangle consists of four cells that occupy exactly two rows, two columns, and two boxes. All four cells have the same two candidates left (in real Sudoku, not all cells have to hold all of the UR candidates).”

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Source: rd.com


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