XY-Wing

Expert

An XY-Wing is three cells with two notes each: a pivot, and two pincers that both see it. Whichever digit the pivot takes, one of the pincers gets their shared digit, so cells that see both pincers can’t have it.

How it works

The pivot can be x or y. One pincer can be x or z, the other y or z. If the pivot is x, the first pincer can’t be x, so it’s z. If the pivot is y, the second pincer is z. So one of the pincers is z for sure, and z goes from every cell that sees both of them.

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The pivot H2 can only be 2 or 3. H5 (3 or 5) and I1 (2 or 5) both see it. If H2 is 3, H5 is 5; if it’s 2, I1 is. Either way one of them is 5, so I5, which sees both, can’t be.

How to spot it

  • Start from a cell with two notes and look at the two-note cells it sees: you need two that each share a different digit with it, and one digit with each other.
  • The cells to clear are where both pincers’ rows, columns and boxes cross. If the pincers share a box or a line, there are more of them.
  • With three notes in the pivot, it’s an XYZ-Wing.

Try it on a real puzzle

Expert puzzles in Sudoku Knockout need techniques like this one, and never a guess. When you’re stuck, a hint shows you the step.

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