Difficulty
Hard
Sudoku technique guide
W-Wing
Sometimes the obvious moves disappear, but the grid is still giving you a useful clue. W-Wing is a calm way to follow it.
By the end, you will know what to notice, how the idea works and when it is worth using.
At a glance
Technique family
Wings and chains
Purpose
Removes candidates
Typical puzzle stage
Late
Frequency
Rare
Prerequisites
Bivalue cells and strong links
Why this technique matters
W-Wing connects two matching bivalue cells through a strong link elsewhere. It is powerful, but the link must be exact.
The idea in plain English
A W-Wing uses two cells with the same pair {X,Y}. A strong link on one of those digits connects units seen by the matching pair cells. This forces one end to be X or the other end to be X, allowing Y to be removed from cells that see both ends.
How to recognise it
The visible cue is a pair of matching bivalue cells that do not share a unit, connected indirectly by a conjugate pair.
Pause when you see this shape. You do not have to force it; just check whether the pattern is complete.
How the logic unfolds
Think of this as a short conversation with the grid. Take one check at a time, and let each answer tell you what to inspect next.
Start by. Find two bivalue cells with the same two candidates.
Then. Choose one candidate and identify a strong link for it between two cells.
Next. Confirm each end of that strong link is seen by one of the matching bivalue cells.
Finally. Remove the other candidate from cells that see both matching cells.
When it usually appears
Use it when normal wing patterns fail but the grid has many strong links for one digit.
Common traps
These are easy places to get turned around. They are useful checks, not signs that you are doing badly.
- Using a weak link instead of a strong link. The forced logic then disappears.
- Eliminating from a cell that does not see both W-Wing endpoints.
Frequently asked questions
What is the strong link doing?
It guarantees one of its two cells contains the chosen digit.
Why are the endpoint pairs identical?
That is what transfers the forced outcome from one endpoint to the other.
Quick takeaway
- A W-Wing belongs to the wings and chains family.
- Its purpose is to remove candidates.
- Look for a complete pattern, not one that only nearly fits.
- It is most useful late.
- Once you make the deduction, return to the simpler scans.