Box/Line Reduction

How It Works

The reverse of Pointing Pairs. When all candidates for a number in a row or column fall within a single 3x3 box, that number can be eliminated from other cells in that box.

Step by Step

1. In a row or column, find a candidate that only appears within one 3x3 box.
2. Since it must go in that box within the row/column, eliminate it from other cells in that box.

When to Use

Use Box/Line Reduction when a candidate in a row or column appears only within one 3x3 box. You can eliminate that candidate from other cells in that box that are not in the row or column.

Example

In row 5, the number 8 is a candidate only in cells that fall within the middle box. So 8 must go in that box within row 5. You can remove 8 from other cells in the middle box that aren't in row 5.

Common Mistakes

Mixing up Box/Line Reduction with Pointing Pairs. Pointing Pairs: look inside a box, eliminate outside. Box/Line: look at a row or column, eliminate inside the box.

Tips

This is the reverse of Pointing Pairs. Scan each row and column; when a candidate is confined to one box, eliminate it from the rest of that box.