W-Wing

Master

A W-Wing is two cells with the same two notes, x and y, that don’t see each other, joined through a line where x has only two places. The two cells can’t both be x, so one of them is y.

How it works

Find a row, column or box where x fits in only two places, one seen by each of the two cells. If both cells were x, neither place could be, and that line would have no x at all. So at least one of the cells is y, and any cell that sees both of them can’t be y.

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C5 and H2 can only be 2 or 9, and don’t see each other. In row A, a 2 fits only in A6 and A2: C5 sees A6, and H2 sees A2 (dashed lines). If both were 2, row A would have no place left for its 2. So one of them is 9, and H5, which sees both, can’t be a 9.

How to spot it

  • Look for two cells with the same two notes far apart. Then, for each of their digits, look for a line with only two notes for it, each seen by one of the cells.
  • It’s a short chain: from one cell’s y to the other’s, through the line’s two places for x.

Try it on a real puzzle

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

Play a Master puzzle