Finned Swordfish is one of the rarest and most advanced fish-pattern techniques in Sudoku. It extends the logic of the standard Swordfish by allowing one of the pattern's corners to have extra candidate cells, known as the "fin." In a normal Swordfish, a candidate digit is confined to at most three columns across exactly three rows (or vice versa). In a Finned Swordfish, one of those rows contains the candidate in an additional column beyond the expected three — these extra cells form the fin. The fin restricts which eliminations are valid: you can only eliminate the candidate from cells that see both the fin's box and the Swordfish's covering columns.
To understand the logic, consider that a standard Swordfish would allow eliminations from all non-base cells in the three cover columns. The fin introduces uncertainty: if the candidate is in the fin cell rather than the expected Swordfish column, the standard elimination would be invalid. However, cells that can see both the fin (by sharing the fin's box) and the Swordfish column are safe to eliminate from regardless of whether the candidate is in the fin or the main pattern. This restricted elimination still provides powerful deductions in puzzles where no standard fish pattern exists.
Finned Swordfish is a last-resort technique that appears in only the most fiendish Sudoku puzzles. It requires a thorough understanding of standard Swordfish patterns, box-line interactions, and the concept of fins from Finned X-Wing. When you encounter a near-perfect Swordfish that is spoiled by one extra candidate cell, do not discard it — check whether the fin allows any restricted eliminations. Mastering this technique demonstrates deep fluency with fish-pattern logic and prepares you for even more exotic variants like Sashimi fish and Kraken fish.
Try It Yourself
Walk through each step of the finned swordfish technique on a real puzzle. Follow the instructions and try entering the correct value when prompted.
We are tracking candidate 3 across the grid. Examine rows 2, 4, and 8 for the positions where 3 can appear. Note which columns contain candidate 3 in each of these rows.
Step-by-Step Guide
Choose a candidate digit and map its positions across all rows of the grid.
Look for three rows where the candidate is mostly confined to three columns — forming a near-Swordfish.
Identify the extra candidate cell that falls outside the three cover columns — this is the fin.
Verify that the fin cell is in the same box as at least one of the main Swordfish cells in that row.
Determine which of the three cover columns intersect with the fin's box.
In those intersecting columns, find cells that are NOT in the three base rows but ARE in the fin's box.
Eliminate the candidate from those cells — they see both the Swordfish column and the fin.
Re-evaluate the grid — the elimination may trigger naked singles or enable further techniques.
Think of a standard three-legged stool that almost balances perfectly, except one leg has a small extra nub sticking out. The stool still works, but the nub limits where you can place it -- only surfaces that accommodate both the leg and the nub are affected.
The Finned Swordfish extends the cover set logic of a standard Swordfish by accounting for the fin cells as an additional constraint. If the digit is placed within the standard Swordfish columns, the normal three-row, three-column cover argument applies and eliminations hold everywhere. If instead the digit occupies a fin cell, the fin is confined to a specific box, so only cells within that box could be affected. Eliminations that are valid under both scenarios -- cells that lie in a cover column AND the fin's box -- are safe regardless of which case is true, making the restricted elimination logically airtight.
When to use: Use Finned Swordfish as a last resort when you spot a near-perfect Swordfish pattern spoiled by one extra candidate cell in a single row. Check whether the fin's box intersects any of the Swordfish columns to find restricted eliminations.
Common Mistakes to Avoid
Applying standard Swordfish elimination rules without accounting for the fin, leading to invalid eliminations.
With a fin present, eliminations are restricted. Only eliminate from cells that can see both the fin's box and the relevant Swordfish cover column. Cells outside the fin's box are unaffected.
Misidentifying which cells form the fin versus the main Swordfish body.
The fin consists of the extra candidate cells in a base row that fall outside the three cover columns. The main body is the candidate cells that fit within the three-row, three-column Swordfish structure.
Overlooking a Finned Swordfish because the pattern is not a clean standard Swordfish.
When a Swordfish is almost valid but one row has an extra candidate, explicitly check for the finned variant before moving to chain-based techniques. The restricted elimination may still unlock progress.
More Examples
See finned swordfish applied in different puzzle configurations to strengthen your pattern recognition.
Finned Swordfish with fin cell
Highlighted cells show the finned swordfish pattern
Practice Puzzles
Apply the finned swordfish technique on these mini challenges. Tap a highlighted cell and enter the correct digit.
Quick Reference
- Pattern:
- A Swordfish 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 Swordfish columns
- Look for:
- A near-perfect Swordfish spoiled by one extra candidate in a box