- Look at the highlighted cells.
- Look at their notes: whichever way they are filled, the same notes are ruled out.
- So 4 can be removed from the crossed-out cells.
W-Wing in sudoku
A W-Wing starts from two cells with the same two notes, A and B, that do not see each other. They are joined by a link: a row, column or box where B can go in only two cells, one seeing the first cell and one seeing the second. Every cell that sees both of the two-note cells loses A.
Or scan with your phone
How to spot it
Look for two cells with identical pairs of notes that are not in the same row, column or box. Then take one of the two numbers and look for a line where it is noted in exactly two cells, each sharing a unit with one of the pair. The other number is the one to remove.
Why it works
The link holds B in one of its two cells. That cell sees one of the pair cells, which then cannot be B and must be A. So at least one of the two pair cells is A, and a cell that sees both cannot be.
On a real puzzle
This is W-Wing 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 W-Wing, 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.