The W-Wing
Learn the W-Wing: two matching bi-value cells {X,Y} joined by a strong link on X, letting you eliminate Y from any cell that sees both.
A W-Wing uses two bi-value cells that share the same two candidates, {X,Y}, but do not see each other. They are bridged by a strong link on one of the digits — a row, column or box where X appears in only two cells, one seeing each {X,Y} cell.
Follow the logic: if either {X,Y} cell is X, the other must become Y; if neither is X, the strong link forces one of them to X anyway. Either way, one of the two cells ends up as Y — so any cell that sees both of them cannot be Y, and you remove it.
The W-Wing is prized because it is symmetric and easy to apply once the strong link is found. The example highlights the two bi-value cells, the connecting strong link and the Y eliminations.
Practise the W-Wing
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
What is a W-Wing in Sudoku?
Two bi-value cells with identical candidates {X,Y}, connected by a conjugate pair on X. One of the two cells must be Y, so cells seeing both lose Y.
What is the "strong link" in a W-Wing?
A unit (row, column or box) where the digit X can go in only two cells — one seeing the first {X,Y} cell and the other seeing the second. It transmits the deduction between the two ends.
How is a W-Wing different from an XY-Wing?
An XY-Wing uses three cells meeting at a pivot. A W-Wing uses two identical bi-value cells linked at a distance by a conjugate pair, so the two ends need not share a cell.
More guides on the Learn hub.