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

Pointing pair

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

An example from a real puzzle

How to spot it

1. Pick a box and a number it still needs.
2. If the number's places in that box are two cells on one row or one column, the box's copy of the number is on that line.
3. Remove the number from the rest of that row or column, outside the box.

Why it works

Whichever of the two cells takes it, the line already has its one copy of the number.