Choose a row or column and a digit that is not yet placed in it.
Box-Line Reduction
The reverse of a pointing pair — a line that pins a digit inside one box.
Box-line reduction, also called claiming or locked candidates type 2, starts from a row or column. If every cell where a digit can go in that line lies inside the same 3×3 box, the digit must be placed in that part of the box. So it cannot appear anywhere else in the box, and you can remove it from the box's other cells.
Worked example
3
3
3
4
6
8
5
3
3
3
3
3
3
3
3
How to apply it
Find every cell in the line where the digit is still possible.
If all of them fall inside one box, the line 'claims' the digit for that box.
Remove the digit from the cells of that box that are not in the line.
When to use it
Box-line reduction rarely places a digit by itself, but it trims candidates so that singles appear. It is a hard-tier technique and pairs naturally with pointing pairs — check both directions whenever you scan a digit.
Frequently asked questions
How is box-line reduction different from a pointing pair?
A pointing pair starts in a box and eliminates along a line. Box-line reduction starts in a line and eliminates inside a box. Both are forms of locked candidates.