- Look at the highlighted cells.
- Look at their notes: whichever way they are filled, the same notes are ruled out.
- So 2 and 7 can be removed from the crossed-out cells.
Unique Rectangle in sudoku
A Unique Rectangle is four empty cells at the corners of a rectangle that spans two rows, two columns and exactly two boxes. Three of the corners hold the same two notes, A and B, and nothing else. The fourth holds A and B and more. A and B can be removed from the fourth corner.
Or scan with your phone
How to spot it
Look for two cells with the same two notes in one box, side by side or one above the other. Follow their rows or columns into another box and look for a third cell with the same pair. The fourth corner completes the rectangle.
Why it works
If the fourth corner were A or B, all four corners would hold only A and B, and the two numbers could be swapped around the rectangle without breaking any row, column or box. The puzzle would have two solutions. A proper sudoku has exactly one, as every puzzle in the game does, so the fourth corner must be something else. The technique is sound only on puzzles with a single solution.
On a real puzzle
This is Unique Rectangle on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
No level of the game needs Unique Rectangle, so this practice puzzle is harder than Extreme, its hardest level. Fill in what you can, then look for the pattern, or ask for the hint: each game has one.