Made by Balthazar Nederlands

Nine

Box line reduction

Nine calls it: Box claimed

The other way around: a digit in a row or column can only go inside one box. Then it is ruled out in the rest of that box.

Example

In column 1 the 8 can only go in the top left box. So the 8 is out everywhere else in the top left box. That leaves one cell with only one digit left: 2.

In the app

Nine is the sudoku that writes this reasoning itself, on every puzzle, in the app. The explanation fills in nothing: it names the technique, colours the proof and says why. Free for seven days, then a one-time unlock. No ads, no account, no internet needed.

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