- Suppose this cell held 7.
- Then these numbers follow, one after another, until a contradiction.
- So 7 is not possible here.
Contradiction in sudoku
Contradiction is the last resort when no pattern is in sight. Take one note in a cell and suppose it were the answer. Fill in the singles that follow from it, one after another. If they lead to something impossible, such as a cell with no number left or the same number twice in a row, the note was wrong and can be removed.
Or scan with your phone
How to spot it
Cells with two notes are the place to start: there are only two things to try, and each one fixes the cell's neighbors quickly. Follow singles only, and stop as soon as something breaks. If nothing breaks, the trial proves nothing, and the note stays.
Why it works
It is a proof, not a guess: a note that leads to an impossible board cannot be part of the solution. The puzzles of the six levels never need it. They are graded by the techniques they take, and a puzzle that could only be solved this way is not given to you.
On a real puzzle
This is Contradiction 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 Contradiction, 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.