Sudoku technique · First needed at: Extreme ●●●●●●●●

Unique rectangle

Three corners of a rectangle carry the same two numbers; the fourth can take neither, or the puzzle would have two answers.

An example from a real puzzle

How to spot it

1. Find four cells in two rows, two columns and two boxes that share the same two candidates.
2. If three of the corners hold only that pair, the fourth cannot be reduced to it.
3. Remove the pair's two numbers from the fourth corner.

Why it works

If all four corners ended as that pair, the two numbers could swap and the puzzle would have two solutions. A proper sudoku has exactly one.