Hard Used from hard puzzles up.
Unique Rectangle
Avoiding a second solution
Four cells at the corners of a rectangle over two rows, two columns and two boxes cannot all end with the same two candidates: the two digits could then swap places, and the puzzle would have two solutions.
So when three corners hold only that pair and the fourth holds more, the fourth cannot be either digit of the pair: remove them, and one of its other candidates is left to it.
This relies on the puzzle having a single solution, as a proper Sudoku does.
What to look for
Find three cells with the same two candidates at three corners of a rectangle spanning two boxes.
Example
- The cells the technique relies on
- Cells where it removes candidates, struck out in red
Practise it
In the app, Learn › Unique Rectangle sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.