Box Line Reduction
The box line reduction technique is an advanced strategy that is used when the possible locations of a number in a row or column are completely within a single region or box.
When to use it
Use when a digit's candidates inside a box all sit on the same row or column.
Worked example
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Inside the box, every candidate for the 3 lies on the same row.
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So the 3 is removed from the rest of that row outside the box.
In practice
For example, if in an upper box the numbers 4 can only appear in cells that are part of row 2, then you can eliminate 4 as a possibility in the other cells of that box. This technique improves efficiency when solving complex sudokus.
Common mistakes
- Confusing the direction: the elimination happens along the line, outside the box, not inside it.
- Applying it when the candidates span two rows or columns — it only works when they share one.
Frequently asked questions
Is a pointing pair the same as box/line reduction?
They are the two sides of the same idea: a pointing pair eliminates along a line from a box; box/line reduction eliminates within a box from a line.