Box-Line Reduction
When all of a digit's candidates in a row or column lie inside a single box, eliminate that digit from the rest of the box.
Intermediate sudoku technique
What it is
Box-Line Reduction is the mirror image of the pointing pair. Where pointing pair operates 'box → line' (eliminating from a line because of constraints inside a box), Box-Line Reduction operates 'line → box' — eliminating from a box because of constraints inside a line. The principle is identical (locked candidates type 2). The naming convention varies by direction so solvers can describe the elimination chain precisely: 'pointing on 5 in box 1' vs 'box-line reduction on 8 from row 4'.
Want the full theory?
The Box-Line Reduction guide on Sudoku247Wiki walks through the logic step by step, with worked examples, diagrams and FAQs.
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