Finned Swordfish in Sudoku: How the Fin Works, With Real Examples

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Published April 17, 2026 · Updated October 14, 2026

A Swordfish needs three rows whose places for a digit all fall in the same three columns. More often you find two rows that qualify and a third that nearly does, spoiled by one stray candidate outside the columns. Don’t throw that one away. If the stray candidate, the fin, sits in a box that one of the three columns runs through, the pattern still removes the digit from that column inside that box. That is a Finned Swordfish.

So we taught our measuring solver to find it and ran the counts. None of our Easy to Expert levels ever needs one. On the first 500 puzzles of our Evil bank it was the simplest move on 104, eight times as many as the plain Swordfish, yet almost every time our Sudoku Helper had a different move ready. Neither the Helper nor the Sudoku Trainer knows the finned version, so this page is where to learn it.

What is a finned swordfish in sudoku?

A pattern on one digit, in two parts:

1. The Swordfish. Three rows hold the digit only in the same three columns, in two or three places each. Call them the base rows and the cover columns.
2. The fin. One of the three rows also holds the digit in one or two cells outside those columns, all inside one box.

The digit then goes from every cell that lies in a cover column and inside the fin’s box, but not in a base row. Elsewhere the cover columns keep their candidates: that is the price of the fin. Rotate it and it still works, with three columns holding the digit in three rows and the fin in one of the columns. The argument is the one our Finned X-Wing guide makes for the smaller fish, with a third row added.

Why can a finned swordfish only remove candidates in the fin’s box?

Here is Evil puzzle 125 after 27 moves. Rows 5 and 9 hold their 6s only in columns 4, 7 and 8. So does row 1, apart from a third 6 in R1C5: the fin. One of two things is true.

• R1C5 is the 6. Then no other cell in box 2 can be, and that includes R3C4.
• R1C5 is not the 6. Then rows 1, 5 and 9 are a plain Swordfish. Each needs its own column for its 6, and between them they have only columns 4, 7 and 8, so each of those columns takes its 6 from one of the three rows. Column 4’s 6 is in R1C4 or R9C4, and R3C4 is out again.

R3C4 loses the 6 either way, so it goes, leaving R3C4 a naked single, 3. The other 6s in columns 7 and 8 are ruled out only in the second case, so they stay.

That caution is not a technicality. In this puzzle the fin is the 6, so row 1 never uses column 7, and column 7 gets its 6 from R8C7, a cell a plain Swordfish would have cleared. Nor is it rare: of the 115 Finned Swordfish our solver found in the first 500 Evil puzzles, the fin was the digit in 56, and in every one of those 56, treating the pattern as a plain Swordfish would have deleted a correct digit. Ignore the fin and about half the time you break the puzzle.

Our Sudoku Helper reaches this exact position with its own hints, then offers an XY-Wing instead, which takes the 6 from R5C7. It knows the plain fish, from the X-Wing up, but no finned ones.

Does the fin have to be next to a corner of the swordfish?

No. It is usually drawn beside one of its own row’s pattern cells, as on puzzle 125, and many guides make that a rule. What the fin actually needs is a cover column running through its box, because that is where the eliminations are. Evil puzzle 236, 13 moves in, has the other kind.

Row 7’s own 3s are in columns 4 and 5, in box 8, and its fin, R7C1, is in box 7. R7C2, where row 7 meets cover column 2, already holds a 1. The fin works through column 2 all the same: either R7C1 is the 3, or the Swordfish holds and column 2’s 3 is in R6C2. Both ways R8C2 and R9C2 lose their 3, and R9C2 is left with 6, the first of the singles that finish the puzzle.

In our count, 17 of the 115 fins shared a box with none of their own line’s pattern cells. In 27, two cover lines crossed the fin’s box, and in 8 of those both of them lost a candidate. So check every cover line that passes through the fin’s box, not just the one beside the fin.

Puzzle 236 also has the twin that our X-Wing guide finds behind every fish. Six rows and six columns are still waiting for a 3. Rows 2, 6 and 7 use columns 2, 4 and 5, so columns 1, 6 and 9 hold their 3s only in rows 4, 8 and 9, apart from the same fin, R7C1. That is a second Finned Swordfish, standing on columns, and it removes the same two 3s.

How do you find a finned swordfish?

Search as for a plain Swordfish, then ask one more question before giving up on a near miss:

1. Pick a digit and write its places row by row, as column numbers. On puzzle 125, row 1 reads 4, 5, 7; row 5, 7, 8; row 9, 4, 8.
2. Find two rows that fit inside three columns and a third that fits apart from one or two extra places. 104 of our 115 had one extra place, 11 had two.
3. Check that the extras share a box and that one of the three columns runs through it. If none does, there is nothing to remove.
4. Look in the fin’s box for the digit in a cover column, outside the three rows. Those candidates go.
5. Then do the columns. 54 of our 115 stood on columns.

How often do you need a finned swordfish?

On our classic Easy, Medium, Hard and Expert levels, never. In 800 boards, 200 per level, nothing beyond naked pairs was ever needed, because a puzzle that singles, locked candidates and naked pairs can’t finish never reaches those levels. The Finned Swordfish belongs to Evil puzzles. We ran our solver over the first 500 of them with 23 techniques, the Helper’s eighteen plus five it lacks, always taking the simplest move. It tries the finned fish straight after the plain ones and before any wing.

The first 500 Evil puzzles, solved simplest first with our 23 techniques. Needed: the number of puzzles where the fish was ever the simplest move. Present: how often, at the 4,618 positions with no single left, the fish would have removed a candidate. Because the finned fish are tried before the wings, they take some positions a wing would also crack. Those 500 head a bank kept in score order, which makes them harder than the bank overall. Measured October 2026.

A fin makes the pattern three times as common as the plain Swordfish: about a third of the positions with no single left hold one somewhere. That makes it a handy check, not an essential one.

Stuck after a finned swordfish, or can’t find one?

After one, look at the cell that lost the candidate, then its row, column and box. On puzzle 125 that was enough: naked singles did the rest. If nothing opens, try what the Helper offered at those same positions: the wings and Almost Locked Sets in the box above.

If you can’t find one, enter the position in the Sudoku Helper and press Next Hint: it won’t show a Finned Swordfish, but it will name the move it would make instead. When the Helper itself runs out, the next stops are X-cycles and XY-chains, with a BUG to check for late in the puzzle and forcing chains held in reserve; our guide to sudoku techniques ranks the lot. To get the plain fish into your eye first, the Trainer has Swordfish drills.