Sudoku You already know the X-Wing — a candidate appearing exactly twice in two rows, with both occurrences aligned in the same two columns. The four cells form a rectangle, and you can eliminate that candidate from the rest of both columns. But what happens when one row has an extra occurrence of the candidate, breaking the perfect pattern?
That extra cell is called the fin. As long as the fin sits in the same box as one of the X-Wing corners, you can still make eliminations — just in a more restricted area. This is a Finned X-Wing, and it appears far more often than the pure version.
In this guide we explain the logic, walk through an example that yields 2 eliminations and 2 naked singles, and show you how to spot Finned X-Wings in your own puzzles.
✅ What Is a Finned X-Wing?
A Finned X-Wing starts like a regular X-Wing: pick a candidate digit that appears in exactly two positions in one row, and nearly the same arrangement in a second row. If the second row has one or more extra occurrences of the candidate — all within the same box as one corner — those extra cells form the fin.
An X-Wing eliminates along entire columns. A Finned X-Wing restricts those eliminations to cells that can see the fin — cells in the same box as the fin and in the X-Wing column. If the X-Wing turns out to be true, normal X-Wing eliminations apply; if the fin cell holds the digit instead, only the box-mates are affected. Either way, the overlapping cells lose the candidate.
🧠 How the Finned X-Wing Works
1. Find a candidate d that appears in exactly two columns in Row A (the clean row).
2. In a second Row B, d appears in those same two columns plus one or more extra cells (the fin).
3. All fin cells must be in the same box as one of the X-Wing corners in Row B.
The Elimination Logic
Consider two scenarios:
- Scenario 1 — The X-Wing is real: Digit d occupies the four corners. Standard X-Wing eliminations apply along both columns.
- Scenario 2 — The fin holds the digit: Digit d is in the fin cell instead of the corner in Row B. Eliminations happen in the fin’s box.
Cells that are eliminated in both scenarios are safe to clear. These are cells that lie in the X-Wing column and share a box with the fin.
Remove candidate d from any cell that is:
• In one of the X-Wing columns, AND
• In the same box as the fin, AND
• Not one of the X-Wing corners or the fin itself.
🔎 Step-by-Step Example
Let’s apply a Finned X-Wing on digit 7 and score 2 eliminations with 2 naked singles.
Step 1 — Find the X-Wing rows
Scan for rows where digit 7 appears as a candidate in just two or three cells:
- Row 3 (clean row): 7 appears in R3C2 = {3, 7} and R3C7 = {5, 7} — exactly two positions.
- Row 8 (finned row): 7 appears in R8C1 = {2, 7, 8}, R8C2 = {6, 7}, and R8C7 = {1, 7} — three positions.
Step 2 — Identify the X-Wing rectangle and the fin
The X-Wing columns are Column 2 and Column 7. The four corners are:
- R3C2 and R3C7 (Row 3)
- R8C2 and R8C7 (Row 8)
The extra cell R8C1 is the fin. It sits in Box 7 (Rows 7–9, Columns 1–3), the same box as the corner R8C2. ✅
Step 3 — Find elimination targets
We eliminate 7 from cells that are in Column 2 (the X-Wing column adjacent to the fin) and in Box 7 (the fin’s box):
- R7C2 = {5,
7} → remove 7 → naked single {5}. - R9C2 = {1,
7} → remove 7 → naked single {1}.
2 eliminations, 2 naked singles — a clean result from a single pattern!
Step 4 — Result
After removing candidate 7 from two cells, both resolve immediately: R7C2 = 5 and R9C2 = 1.
🔄 Finned X-Wing vs. Other Techniques
| Feature | X-Wing | Finned X-Wing | Swordfish |
|---|---|---|---|
| Pattern shape | Perfect 2×2 rectangle | 2×2 rectangle + fin cell(s) | 3×3 grid (up to 6 cells) |
| Rows / columns | 2 rows, 2 columns | 2 rows, 2 columns + fin in one box | 3 rows, 3 columns |
| Elimination scope | Entire X-Wing columns | Only cells seeing the fin and X-Wing column | Entire Swordfish columns |
| Frequency | Uncommon | More common than pure X-Wing | Rare |
| Difficulty | Intermediate | Intermediate–Advanced | Advanced |
🕵️ How to Spot a Finned X-Wing
1. Pick a candidate digit d.
2. Find a row where d appears in exactly two cells — this is your clean row.
3. Find a second row where d appears in those same two columns plus extra cells — this is your finned row.
4. Verify the extra cells (fins) are all in the same box as one corner in the finned row.
5. Eliminate d from cells in the X-Wing column that share a box with the fin.
Tip: You can also base the X-Wing on columns instead of rows — the logic is identical, just rotated 90°.
⚠️ Common Mistakes
1. Eliminating from the entire column
Unlike a regular X-Wing, a Finned X-Wing only allows eliminations in the fin’s box. Removing the candidate from the full column is incorrect.
2. Fin not in the same box as a corner
If the extra candidate cell is in a different box from both corners in that row, the pattern is not a valid Finned X-Wing.
3. Forgetting to check both columns
The fin is adjacent to one corner. Eliminations happen in that corner’s column only (where the fin’s box overlaps). The other column gets no eliminations.
4. Confusing Finned X-Wing with Sashimi X-Wing
A Sashimi X-Wing is similar but one corner is missing entirely, with the fin compensating. In a Finned X-Wing, all four corners exist plus the fin.
5. Overlooking column-based patterns
Finned X-Wings can be based on columns too. The clean column has the candidate in exactly two rows; the finned column has it in those rows plus extra cells in one box.
📅 When to Look for Finned X-Wings
- Basic: Naked Singles, Hidden Singles.
- Intermediate: Naked Pairs, Pointing Pairs, X-Wing.
- Intermediate–Advanced: Finned X-Wing, Skyscraper, XY-Wing.
- Advanced: Swordfish, Simple Colouring.
- Expert: 3D Medusa, Sue de Coq.
🚀 Beyond Finned X-Wings
| Technique | What It Adds | Complexity |
|---|---|---|
| X-Wing | Perfect 2×2 rectangle, eliminates along full columns | Intermediate |
| Finned X-Wing | X-Wing + fin cell, restricted elimination zone | Intermediate–Advanced |
| Skyscraper | Two conjugate pairs sharing one endpoint column | Intermediate–Advanced |
| Swordfish | 3×3 fish pattern | Advanced |
| Simple Colouring | Conjugate-pair chains for one digit | Advanced |
🎯 Practice Finned X-Wings
Sudoku Hard
Hard puzzles frequently require Finned X-Wings and other advanced fish techniques.
▶ Play Sudoku HardX-Wing Guide
Make sure you’ve mastered the basic X-Wing — the foundation the Finned version builds on.
▶ Read the X-Wing GuideSudoku Solver
Paste any puzzle and watch the solver identify techniques step by step.
▶ Open the SolverFrequently Asked Questions
A Finned X-Wing is a variation of the X-Wing where one corner has extra candidate cells (the fin) in the same box. Eliminations are restricted to cells that see both the fin and the X-Wing column.
A regular X-Wing eliminates along the full length of both columns. A Finned X-Wing has an extra occurrence (the fin) that limits eliminations to cells sharing a box with the fin.
Only in cells that are in the X-Wing column and share a box with the fin cell. This is the intersection of the normal X-Wing elimination zone and the fin’s box.
The fin is the extra cell in one of the base rows that contains the target candidate but falls outside the X-Wing rectangle. It must be in the same box as one of the X-Wing corners in that row.
After basic techniques are exhausted. When you see a candidate forming a near-perfect X-Wing rectangle with just one extra cell, check if that extra cell is in the same box as a corner and apply the Finned X-Wing.