The Finned X-Wing is a variation of the standard X-Wing where one of the two rows has the candidate in additional cells beyond the two X-Wing columns. These extra cells are called the "fin." In a regular X-Wing, a candidate appears in exactly two cells in each of two rows, aligned in the same two columns. In a Finned X-Wing, one row has the candidate in exactly two cells (the "base row"), but the other row has it in two or more cells — two in the X-Wing columns plus extras in the same box as one of the X-Wing corners.
The fin limits the power of the elimination but does not destroy it entirely. The logic is: either the standard X-Wing pattern holds (the fin cells are not the solution) and the normal column eliminations apply, or one of the fin cells is the solution. In either case, eliminations can still be made from cells that would be eliminated by the X-Wing AND that can also see all the fin cells. In practice, this means the eliminations are restricted to the intersection of the X-Wing column and the box containing the fin.
The Finned X-Wing appears more frequently than the standard X-Wing because the "exactly two per row" requirement is relaxed for one row. It is an important bridge technique between basic fish patterns and their more complex relatives like Finned Swordfish and Franken Fish. Mastering it teaches you how imperfect patterns can still yield useful eliminations, a principle that applies throughout advanced Sudoku solving.
Try It Yourself
Walk through each step of the finned x-wing technique on a real puzzle. Follow the instructions and try entering the correct value when prompted.
We are looking for a Finned X-Wing on candidate 5. In row 1, candidate 5 appears in cells (0,1) and (0,4) — exactly two positions. In row 4, candidate 5 appears in (3,1), (3,4), and (3,5). The extra cell (3,5) is the fin.
Step-by-Step Guide
Choose a candidate digit and find rows where it appears in two or three cells.
Look for two rows where the candidate aligns in the same two columns, but one row has extra candidate cells (the fin).
Verify the fin cells are in the same box as one of the X-Wing corners in that row.
Identify the X-Wing column that passes through the fin's box.
Eliminate the candidate from cells in that column that are inside the fin's box but outside the X-Wing rows.
Check the grid for new singles or patterns enabled by the elimination.
It is like an X-Wing with a small tear in one corner — the tear limits where you can patch, but you can still fix cells that are visible to both the pattern and the torn spot.
The logic rests on a disjunction: either the fin cells do not contain the digit (in which case the pattern reduces to a standard X-Wing and all normal column eliminations hold), or a fin cell does contain the digit (in which case the digit is placed within the fin's box). In both branches, any cell that lies in the X-Wing elimination column AND shares the fin's box is excluded — in the first branch by X-Wing logic, and in the second branch by box uniqueness. Since the elimination holds under every possible case, it is sound despite the imperfect pattern.
When to use: Use Finned X-Wing when you almost have an X-Wing but one row has extra candidate cells (the fin) in the same box as an X-Wing corner. Eliminations are restricted to cells in the fin's box that also lie in the X-Wing column.
Common Mistakes to Avoid
Applying full X-Wing eliminations across the entire column when a fin is present.
When a fin exists, eliminations are restricted to cells that can see both the X-Wing column AND all fin cells. This typically limits eliminations to the intersection of the column and the fin's box.
Treating extra candidate cells outside the fin's box as part of the fin.
The fin must be in the same box as one of the X-Wing corners in that row. Extra cells in other boxes break the pattern entirely.
Ignoring Finned X-Wings because they seem weaker than regular X-Wings.
Finned X-Wings occur much more frequently than regular X-Wings. Even one or two restricted eliminations can unlock the next step of the puzzle.
More Examples
See finned x-wing applied in different puzzle configurations to strengthen your pattern recognition.
Finned X-Wing on Digit 2
Highlighted cells show the finned x-wing pattern
Practice Puzzles
Apply the finned x-wing technique on these mini challenges. Tap a highlighted cell and enter the correct digit.
Quick Reference
- Pattern:
- An X-Wing pattern with one extra candidate cell (the fin) in one row
- Action:
- Eliminate the candidate from cells that see both the fin's box and the X-Wing columns
- Look for:
- A near-perfect X-Wing spoiled by one extra candidate in a box