Sudoku-YAdvanced
W-Wing: matching pairs joined by a bridge
Two distant {3,8} cells are not a naked pair. A forced bridge can make them useful together.
The bridge is essential
A W-Wing connects matching bivalue cells through a strong link for one of their digits. At least one endpoint must then contain the other digit, which can be removed from cells seeing both endpoints.
Our example: bridge on 8, eliminate 3
Let B2={3,8} and H8={3,8}. Suppose column 5 has no placed 8 and its only positions for 8 are B5 and H5. B2 sees B5; H8 sees H5. If both endpoints were 8, neither bridge cell could be 8, leaving column 5 without a home for it.
At least one endpoint must therefore be 3. B8 sees both endpoints through row B and column 8, so remove its 3. H2 is another target through column 2 and row H.
Break the bridge to test the reason
Add an 8 candidate at E5. Both endpoints can now be 8 while column 5 places its 8 at E5. The original argument no longer removes 3 at B8. It does not establish an elimination of 8 there either.
Keep the two digit roles separate
Here 8 supplies the bridge and 3 is the target elimination. Say “what if both endpoints were 8?” before acting. That short contradiction is easier to audit than remembering the outline alone.
In Classic 9×9, use Scan to scan the current entries, inspect justified eliminations and open this lesson. Chain and ALS searches are bounded; UR Type 1 and BUG+1 require verified uniqueness. Reading this lesson never changes your saved game.
Try before looking
Try before revealing
You find B2 and H8 both holding {3,8}, but have not checked column 5. Is that enough?
Show explanation
No. Verify the forced bridge and the endpoint-to-bridge connections first. Matching pairs alone do not establish a W-Wing.
References and further reading
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