Sudoku-YAdvanced
Finned X-Wing: an extra candidate narrows the targets
An extra candidate prevents an ordinary X-Wing, but some targets may still be blocked in every case.
The fin limits the elimination area
In the simple pattern here, a target must lie in a column blocked by the underlying fish and in the box blocked by the fin. We prove both cases rather than applying ordinary X-Wing eliminations everywhere.
Our fin at H9
Let row B allow 6 only at B2/B8 and row H only at H2/H8/H9, with no placed 6 in either row. H9 is the extra candidate. Target G8 belongs to column 8 and the bottom-right box.
- If H9=6, its box blocks G8.
- If H9≠6, B2/B8/H2/H8 form an X-Wing that blocks G8 through column 8.
Remove 6 at G8 without settling H9 first.
Why C8 is different
C8 shares column 8 but cannot see the fin. The partial premises still permit H9=6, B2=6 and C8=6 together. Do not clear the entire column. More fins outside the stated box would also require a fresh proof.
Audit the two branches
Ask “what blocks the target when the fin is true?” and “what remains when it is false?” Both answers are necessary. This article covers one simple fin arrangement, not every variety of finned fish.
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
A target sees the fin but is outside the underlying X-Wing columns. Is that sufficient?
Show explanation
No. The false-fin case must block it as well. Seeing the fin establishes only one branch.
References and further reading
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