The Finned X-Wing is a clever twist on the standard X-Wing. It’s what happens when you almost have a perfect X-Wing, but one extra little group of candidates (the "Fin") is spoiling the pattern.
Instead of giving up, you can use that "Fin" to make a very specific elimination.
What is a "Fin"?
Imagine you are looking for an X-Wing (Col) for the number 3. You find two columns that form a near-perfect rectangle, but in one of those columns, there is an extra "3" sitting right next to one of the corners, inside the same 3x3 box.
That extra cell is the Fin. It breaks the perfect X-Wing, but it also creates a new opportunity.
Interactive Example
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Click "Apply Logic" to see the strategy in action.
Real Example Explanation
In the interactive example above, focus on the number 3:
- The Base Pattern: Look at Column 5 and Column 9.
- The Box Restriction: In Row 3 and Row 9, we almost have a perfect rectangle. However, R1C5 (Green) is an extra 3 in Column 5.
- The "Win-Win" Scenario:
- If the Fin is False: Then the remaining candidates form a perfect X-Wing. This would eliminate any 3 in Row 3 (the target row).
- If the Fin is True: Then the Fin is the 3 for that box. Any other 3 in that same 3x3 Box must be eliminated.
- The Magic Spot: The strategy works by finding cells that are attacked by BOTH possibilities.
- The cell R3C4 (Red) is in Row 3 (attacked by the X-Wing).
- The cell R3C4 is also in Box 2 (attacked by the Fin).
- Therefore, R3C4 cannot be a 3.
A Simple Visual Summary
- X-Wing: A perfect rectangle. Deletes across the whole row.
- Finned X-Wing: A rectangle with a "little extra" in one box. Deletes only where the row and the box overlap.
How to Spot It
- Find a "Broken" X-Wing: Find two vertical columns where a number appears twice in one (the "straight" side) and 2-3 times in the other.
- Check the Alignment: The extra spots (the Fin) must all be in the same 3x3 box as one of the corners.
- Identify the Target: Look at the row that lines up with the Fin. Follow that row into the 3x3 box where the Fin lives.
- Delete: Any candidate in that row, inside that specific box, can be removed.
Related Strategies
- X-Wing (Col): The perfect version without fins.
- Sashimi X-Wing: A version where the corner itself is missing, leaving only the fin!