Finned X-Wing
A Finned X-Wing is an X-Wing that almost works — it has one extra candidate spoiling the clean pattern. That extra candidate is called the fin, and far from ruining the technique, it just narrows where the eliminations land. Finned fish turn up much more often than perfect ones, so this is a high-value pattern to know.
What is a fin?
Recall a clean X-Wing: a digit sits in exactly two cells of two rows, sharing the same two columns, and it's eliminated from the rest of those columns. A Finned X-Wing is the same, except one of the four "corner" rows has an extra cell where the digit could also go — usually within the same box as one corner. That extra cell is the fin.
How the eliminations shrink
With a clean X-Wing you eliminate the digit throughout both columns. With a fin, the logic still holds — but only for cells that would be attacked whether or not the fin is the true placement. In practice that means the eliminations survive only in cells that see both the normal X-Wing cover and the fin — typically the cells in the fin's own box. So instead of clearing a whole column, you clear a few cells near the fin.
A near X-Wing on 6: base rows 2 and 8, cover columns 4 and 7 (blue). The extra 6 at r8c5 (orange) is the fin. Only r7c4 — a cover cell sharing box 8 with the fin — is safe to clear.
Step by step
- Look for a near X-Wing — a digit almost forming the two-row, two-column rectangle.
- Spot the fin — the one extra cell that breaks the clean pattern, usually in a corner box.
- Keep only the safe eliminations — cover cells that also see the fin (its box).
Sashimi fins
A special case, the Sashimi X-Wing, is a finned fish where one of the "real" corner cells is missing entirely, leaving only the fin to do the work. It looks broken but the eliminations still hold by the same reasoning. Finned and Sashimi fish scale up too: the Finned Swordfish applies identical logic to the order-3 fish.
Common mistakes
- Keeping too many eliminations. Only cells that see the fin as well as the cover survive — don't clear the whole column.
- Multiple fins in different boxes. If the extra candidates spread across boxes, there's no single safe elimination.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.