Sudoku technique · First needed at: Medium ●●●○○○○○

Box-line reduction

Along this line the number sits in a single box, so it can leave that box elsewhere.

An example from a real puzzle

How to spot it

1. Pick a row or column and a number it still needs.
2. If every place for the number on that line falls inside one box, the line's copy is in that box.
3. Remove the number from the box's other cells.

Why it works

The box can hold the number only once, and the line has already claimed it.