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

Empty rectangle

Inside this box the number sits on one row and one column; pair that with a two-spot line to clear the crossing.

An example from a real puzzle

How to spot it

1. Find a box where a number's places all lie on one row and one column of the box, like a cross.
2. Find a line elsewhere where the number has only two places, one of them in line with the cross.
3. The cell where the other end of that line meets the cross's other arm loses the number.

Why it works

If the line's far end is false, its near end is true and blocks one arm of the cross, pushing the number onto the other arm. Either way the target cell is covered.