Empty Rectangle
A box where a digit's candidates lie entirely along one row or one column, plus a strong link on that digit elsewhere, force an elimination on the intersection cell.
Advanced sudoku technique
What it is
An Empty Rectangle (ER) is a 3×3 box where digit X's candidates are confined to exactly ONE row OR ONE column inside the box — the rest of the box has no X candidates. (The 'rectangle' is the empty 2×3 or 3×2 region inside the box that DOESN'T hold any X.) Combine the ER with a strong link on X somewhere else: a row or column outside the box where X fits in only two cells, and one of those cells sits in the ER's column-of-confinement (or row-of-confinement). The chain: that strong-link cell is X, or the other end of the strong link is X. Either way, the elimination cell — the intersection of the strong link's outer end with the ER's confinement axis — drops X.
Want the full theory?
The Empty Rectangle guide on Sudoku247Wiki walks through the logic step by step, with worked examples, diagrams and FAQs.
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