Sudoku You already know the Swordfish — a candidate appearing in at most two positions in each of three rows, with all occurrences confined to the same three columns. The pattern lets you eliminate that candidate from any other cell in those three 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 Swordfish pattern cells in that row, you can still make eliminations — just in a more restricted area. This is a Finned Swordfish, and it appears considerably more often than the pure version.
In this guide we explain the logic, walk through an example that yields 2 eliminations and a naked single, and show you how to spot Finned Swordfish patterns in your own puzzles.
✅ What Is a Finned Swordfish?
A Finned Swordfish starts like a regular Swordfish: pick a candidate digit that appears in at most two positions in each of three rows, with all those positions confined to the same three columns. If one of those rows has an extra occurrence of the candidate — in the same box as one of the Swordfish pattern cells in that row — those extra cells form the fin.
A regular Swordfish eliminates along entire columns. A Finned Swordfish restricts those eliminations to cells that can see the fin — cells in the same box as the fin and in one of the Swordfish columns. If the Swordfish is true, normal 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 Swordfish Works
1. Find a candidate d that forms a Swordfish pattern across three rows (each row has d in at most two of the same three columns).
2. One of those rows has an extra occurrence of d outside the three Swordfish columns.
3. That extra cell (the fin) must be in the same box as one of the Swordfish pattern cells in that row.
The Elimination Logic
Consider two scenarios:
- Scenario 1 — The Swordfish is real: Digit d occupies the six pattern cells. Standard Swordfish eliminations apply along all three columns.
- Scenario 2 — The fin holds the digit: Digit d is in the fin cell instead of the pattern cell in that row. Eliminations happen in the fin’s box.
Cells that are eliminated in both scenarios are safe to clear. These are cells that lie in a Swordfish column and share a box with the fin.
Remove candidate d from any cell that is:
• In one of the Swordfish columns, AND
• In the same box as the fin, AND
• Not one of the Swordfish pattern cells or the fin itself.
🔎 Step-by-Step Example
Let’s apply a Finned Swordfish on digit 5 and score 2 eliminations including a naked single.
Step 1 — Find the Swordfish rows
Scan for rows where digit 5 appears as a candidate in two or three cells confined to specific columns:
- Row 2: 5 appears in R2C1 = {1, 5} and R2C4 = {4, 5} — two positions in Columns 1 and 4.
- Row 5: 5 appears in R5C1 = {1, 5} and R5C4 = {1, 5, 9} — two positions in Columns 1 and 4.
- Row 8: 5 appears in R8C1 = {1, 5}, R8C2 = {4, 5, 9}, and R8C7 = {5, 7} — three positions.
Step 2 — Identify the Swordfish pattern and the fin
The three Swordfish columns are Column 1, Column 4, and Column 7. The six pattern cells are:
- R2C1 and R2C4 (Row 2)
- R5C1 and R5C4 (Row 5)
- R8C1 and R8C7 (Row 8)
The extra cell R8C2 is the fin. It sits in Box 7 (Rows 7–9, Columns 1–3), the same box as the pattern cell R8C1. ✅
Step 3 — Find elimination targets
We eliminate 5 from cells that are in Column 1 (the Swordfish column within the fin’s box) and in Box 7 (the fin’s box), excluding the pattern cell R8C1:
- R7C1 = {1,
5} → remove 5 → naked single {1}. - R9C1 = {5, 6, 8} → remove 5 → {6, 8}.
2 eliminations, 1 naked single — a clean result from a single pattern!
Step 4 — Result
After removing candidate 5 from two cells, R7C1 resolves immediately as a naked single: R7C1 = 1. R9C1 is reduced to {6, 8}, simplifying further deductions.
🔄 Finned Swordfish vs. Other Techniques
| Feature | Swordfish | Finned Swordfish | Finned X-Wing |
|---|---|---|---|
| Pattern shape | 3×3 grid (up to 6 cells) | 3×3 grid + fin cell(s) | 2×2 rectangle + fin cell(s) |
| Rows / columns | 3 rows, 3 columns | 3 rows, 3 columns + fin in one box | 2 rows, 2 columns + fin in one box |
| Elimination scope | Entire Swordfish columns | Only cells seeing the fin and Swordfish column | Only cells seeing the fin and X-Wing column |
| Frequency | Rare | More common than pure Swordfish | Fairly common |
| Difficulty | Advanced | Advanced–Expert | Intermediate–Advanced |
🕵️ How to Spot a Finned Swordfish
1. Pick a candidate digit d.
2. Find three rows where d appears in positions that mostly align in the same three columns.
3. Verify that at most two positions per row fall within those three columns. If one row has an extra position outside those columns, that’s the finned row.
4. Check that the extra cell (fin) is in the same box as one of the Swordfish pattern cells in that row.
5. Eliminate d from cells in the Swordfish column that share a box with the fin.
Tip: You can also base the Swordfish on columns instead of rows — the logic is identical, just rotated 90°.
⚠️ Common Mistakes
1. Eliminating from entire Swordfish columns
Unlike a regular Swordfish, a Finned Swordfish 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 pattern cell
If the extra candidate cell is in a different box from all pattern cells in that row, the pattern is not a valid Finned Swordfish.
3. Eliminating from the wrong column
The fin is adjacent to one pattern cell. Eliminations happen only in the column of that adjacent pattern cell where it overlaps with the fin’s box. Other Swordfish columns get no eliminations.
4. Confusing with Sashimi Swordfish
A Sashimi Swordfish is similar but one pattern cell is missing entirely, with the fin compensating. In a Finned Swordfish, all six pattern cells exist plus the fin.
5. Overlooking column-based patterns
Finned Swordfish can be based on columns too — three columns with aligned occurrences in three rows, plus a fin. Don’t only check rows.
📅 When to Look for Finned Swordfish
- Basic: Naked Singles, Hidden Singles.
- Intermediate: Naked Pairs, Pointing Pairs, X-Wing.
- Intermediate–Advanced: Finned X-Wing, Skyscraper, XY-Wing.
- Advanced: Swordfish, Finned Swordfish, Simple Colouring.
- Expert: 3D Medusa, Sue de Coq.
🚀 Beyond Finned Swordfish
| Technique | What It Adds | Complexity |
|---|---|---|
| Swordfish | Perfect 3×3 fish pattern, eliminates along full columns | Advanced |
| Finned Swordfish | Swordfish + fin cell, restricted elimination zone | Advanced–Expert |
| Finned X-Wing | X-Wing + fin cell, restricted elimination zone | Intermediate–Advanced |
| Jellyfish | 4×4 fish pattern | Expert |
| 3D Medusa | Multi-digit colouring chains | Expert |
🎯 Practice Finned Swordfish
Sudoku Hard
Hard puzzles frequently require Finned Swordfish and other advanced fish techniques to crack.
▶ Play Sudoku HardSwordfish Guide
Make sure you’ve mastered the basic Swordfish — the foundation the Finned version builds on.
▶ Read the Swordfish GuideSudoku Solver
Paste any puzzle and watch the solver identify fish techniques step by step.
▶ Open the SolverFrequently Asked Questions
A Finned Swordfish is a variation of the Swordfish where one row has extra candidate cells (the fin) in the same box as a pattern cell. Eliminations are restricted to cells that see both the fin and the Swordfish column.
A regular Swordfish eliminates along the full length of all three columns. A Finned Swordfish has an extra occurrence (the fin) that limits eliminations to cells sharing a box with the fin.
Only in cells that are in a Swordfish column and share a box with the fin cell. This is the intersection of the normal Swordfish 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 Swordfish pattern. It must be in the same box as one of the Swordfish pattern cells in that row.
After intermediate techniques are exhausted. When you see a candidate forming a near-perfect Swordfish with just one extra cell in a row, check if that extra cell is in the same box as a pattern cell and apply the Finned Swordfish.