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'.
When to use it
When scanning shows a digit's candidates in some row or column all clustered into a single box. Use it to clear that digit from the rest of the box.
Worked example
Digit 3's only candidate cells in row 7 are R7C4 and R7C6 — both in box 8 (the bottom-middle box). The 3 in row 7 must land somewhere in box 8. Therefore 3 can be eliminated from R8C4, R8C5, R8C6, R9C4, R9C5, R9C6 (the rest of box 8 outside row 7).
Try it
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