Nine
X-wing
A technique Nine explains
A digit fits in only two columns in each of two rows, and they are the same two columns. The four cells form a rectangle. Each row holds the digit in one of those columns, so it is ruled out everywhere else in them. It works with rows and columns swapped as well.
Example
In row 1 and row 6 the 3 can only go in the same two columns. Those four cells form a rectangle: each line holds the 3 in one of the two, so it is out everywhere else in column 2 and column 6. That leaves one cell with only one digit left: 1.
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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