Sudoku techniques · Master
W-Wing in Sudoku
A W-Wing is two bivalue cells holding the same pair of candidates, AB, connected by a strong link on A whose two ends each see a different one of the cells. The link forces at least one of the two cells to be B, so any cell seeing both of them loses B.
Why it works
Two matching bivalue cells share two candidates. A strong link on one of the two candidates, with each end seeing a different one of the cells, forces at least one matching cell to hold the other candidate, so any cell that sees both loses it.
How to spot it
Start by collecting bivalue cells with matching candidate pairs. The board usually has a few.
- For each matching pair of cells, take one of their two candidates and look for a unit where that number has exactly two spots.
- One end of that strong link must see one cell, the other end the other cell.
- The elimination is on the other candidate: the one the strong link is not on. That reversal is what makes W-Wings feel counter-intuitive.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
Two matching bivalue cells have the candidates 3 and 7. The two related cells form a strong link on 7, each seeing a different matching cell. Whichever related cell holds 7 rules that candidate out of the matching cell it sees, forcing that matching cell to take the other candidate. This is a W-Wing!
- 2
At least one of the two matching bivalue cells must be 3, so we can remove the candidate 3 from any other cell that sees both matching cells.