Box/line reduction

This is pointing in reverse. If every place a number can go in a row (or column) lies inside a single box, then that number must sit in that box on that line. It can’t appear anywhere else in the box.

How to spot it

1. Pick a row or column and a number it still needs. Mark where it can go.
2. If all the marks are inside one box, the number will end up in that box on this line.
3. Remove the number from the box’s other cells.

A real example

In col 1, 8 can only go in r5c1 and r6c1, all inside box 4. So box 4’s 8 is in this col, and the other cells of the box can’t be 8: r5c2 loses 8.

Tips

• If pointing pairs are the box telling the line what to do, this is the line telling the box.
• Check rows and columns that have only two or three empty cells left.

Practice on Medium puzzles