Box-Line Reduction
A named case of locked candidates where a line restricts a digit to one box, eliminating it from that box's other cells. The backbone of hard-level pruning.
Candidate 4 in this column only fits these two cells — both in the top box.
How to spot it
- 1Find a row or column where a digit's candidates all fall in one box.
- 2The digit must be placed within that intersection.
- 3Remove it from the box's cells outside the line.
When to use
Whenever a line's candidates cluster inside a single box.
Common mistake
Forgetting to re-scan the box for new singles the reduction just created.
Worked Example
Box-line reduction is the named case where a line — not a box — does the confining. If every candidate for 4 in a column falls inside a single box, the 4 belonging to that column has to be placed within that box.
That means the rest of the box, outside the column, can't also hold a 4, so those candidates come off. It's the same underlying idea as locked candidates generally, just specifically the line-confines-a-box direction — and it's worth re-scanning the box afterward, since clearing those candidates often reveals a fresh single.