Fish & Wings
Finned Swordfish
A Finned Swordfish is a Swordfish with one flaw: one base line also has the digit outside the three cover lines, in one or two cells of a single box — the fin. Either the fin holds the digit, or it does not and the Swordfish is real. A cell that loses the digit in both cases — a cover-line cell in the fin's box, outside the base lines — can lose it now.
What you need to see on the grid
Look for a Finned Swordfish when a Swordfish almost works: three lines fit three cross-lines except for a stray candidate or two. Four terms carry the method:
- Base and cover lines
- As in a Swordfish: three base lines whose places for the digit fall in three cover lines of the other direction — here columns 2, 4 and 7 over rows 4, 5 and 9.
- The fin
- The place, or places, of the digit in a base line that miss the cover lines. They must all be in one base line and one box.
- The two cases
- If the fin holds the digit, its box is used up. If it does not, the three base lines form a true Swordfish and clear the cover lines.
- The targets
- Cover-line cells inside the fin's box, outside the base lines: the only cells both cases clear.
How to apply it, step by step
One example runs through all five steps: the digit 3, in an expert puzzle from the app, at the exact point where its hint needs a Finned Swordfish. Every diagram shows the whole grid. The cells a step needs are filled in; the others are left blank and hatched, so you know you can ignore them. A digit missing from a drawn cell is already placed in its row, column or box, drawn or not.
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Step 1Spot a Swordfish spoiled by an extra place
List the places of a digit column by column, as for a Swordfish. For 3, column 2 has R4C2, R5C2 and R8C2; column 4 has R4C4, R5C4 and R9C4; column 7 has R4C7 and R5C7.
All but one of those cells sit in rows 4, 5 and 9 — a Swordfish, were it not for R8C2. That extra place is the fin.
Columns 2, 4 and 7. Their 3s fit rows 4, 5 and 9, except R8C2. C1 C2 C3 C4 C5 C6 C7 C8 C9 R1 2 9 6 R2 1 R3 7 6 1 R4 R5 R6 4 7 2 R7 R8 5 7 R9 1 - Digit already on the grid
- Base columns
- A place for 3 in a base column (R8C2 is the fin)
- Candidate 3, the digit studied
- Not needed for this step
Filled in: columns 2, 4 and 7. The other 54 cells are left blank: they are not needed for this step. Nothing is removed yet: this step only reads the grid.
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Step 2Check that the fins sit in one box
A fin is only useful if all fins share one box and one base line. Here there is a single fin, R8C2, in box 7.
Box 7 is also crossed by a cover row: row 9 runs through R9C1, R9C2 and R9C3. Those crossing cells are where the move can happen; a fin whose box no cover line crosses gives nothing.
The base columns and box 7, which holds the fin R8C2 and is crossed by row 9. C1 C2 C3 C4 C5 C6 C7 C8 C9 R1 2 9 6 R2 1 R3 7 6 1 R4 R5 R6 4 7 2 R7 7 2 R8 6 4 5 7 R9 1 - Digit already on the grid
- Base columns
- A place for 3 in a base column (R8C2 is the fin)
- Candidate 3, the digit studied
- Not needed for this step
Filled in: columns 2, 4 and 7; box 7. The other 48 cells are left blank: they are not needed for this step. Nothing is removed yet: this step only reads the grid.
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Step 3Reason in two cases: a fin holds the digit, or the Swordfish is real
Case 1: R8C2 is 3. Then box 7 has its 3, and no other cell of box 7 can be 3.
Case 2: R8C2 is not 3. Then columns 2, 4 and 7 keep their 3s in rows 4, 5 and 9, one column per row: a true Swordfish, which removes 3 from every other cell of those three rows — R4C5, R5C5, R5C6, R9C1 and R9C3.
The whole fish: base columns 2, 4, 7, cover rows 4, 5, 9, and the fin's box 7. C1 C2 C3 C4 C5 C6 C7 C8 C9 R1 2 9 6 R2 1 R3 7 6 1 R4 2 7 1 6 R5 1 5 7 R6 4 7 2 R7 7 2 R8 6 4 5 7 R9 1 6 7 4 - Digit already on the grid
- Base columns
- Cover rows
- A place for 3 in a base column (R8C2 is the fin)
- Candidate 3, the digit studied
- Not needed for this step
Filled in: rows 4, 5 and 9; columns 2, 4 and 7; box 7. The other 32 cells are left blank: they are not needed for this step. Nothing is removed yet: this step only reads the grid.
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Step 4Remove the digit only where both cases agree
A cell loses 3 for good only if it loses it in both cases: it must be in a cover row (case 2) and in box 7 (case 1), outside the base columns. That leaves R9C1 and R9C3. 3 leaves both.
R4C5, R5C5 and R5C6 keep their 3: they lose it only in case 2, and case 1 — the fin being 3 — is still possible.
Technique: Finned Swordfish
Row 9 crosses box 7 at R9C1 and R9C3, outside the base columns. 3 leaves both. C1 C2 C3 C4 C5 C6 C7 C8 C9 R1 2 9 6 R2 1 R3 7 6 1 R4 2 7 1 6 R5 1 5 7 R6 4 7 2 R7 7 2 R8 6 4 5 7 R9 1 6 7 4 - Digit already on the grid
- Base columns
- Cover rows
- A place for 3 in a base column (R8C2 is the fin)
- Candidate 3, the digit studied
- Candidate removed
- Not needed for this step
Filled in: rows 4, 5 and 9; columns 2, 4 and 7; box 7. The other 32 cells are left blank: they are not needed for this step. Removed: 3 from R9C1 and R9C3.
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Step 5Collect what the elimination unlocks
Look at box 7 again. Its places for 3 were R8C2, R9C1 and R9C3. With the last two gone, R8C2 is the only one left: a Hidden Single, and the app's next hint in this puzzle. The fin turns out to be the 3 after all.
Technique: Hidden Single
Box 7 after the Finned Swordfish. 3 now fits only R8C2. C1 C2 C3 C4 C5 C6 C7 C8 C9 R1 R2 R3 R4 R5 R6 R7 7 2 R8 6 3 4 R9 1 - Digit already on the grid
- Box 7: one place left for 3
- The cell that must be 3
- Candidate 3, the digit studied
- Digit placed
- Not needed for this step
Filled in: box 7. The other 72 cells are left blank: they are not needed for this step. Placed: R8C2 = 3.
Common mistakes
Clearing the cover lines as if there were no fin
With a fin, the Swordfish is only one of two cases. Removing 3 from R4C5, R5C5 or R5C6 assumes the fin is not 3 — which nobody has shown. Only the cells in the fin's box are safe.
Fins in two boxes or two lines
Case 1 only works when every fin sees the targets, so all fins must share one box — and one base line, or two fins could hold the digit at once. Fins spread wider prove nothing.
Clearing the whole fin's box
Case 2 says nothing about the fin's box outside the cover lines. A cell of box 7 in row 7 or row 8 would lose the digit only in case 1, so it would keep it. In this puzzle those cells are already filled; in others they are not, and they stay as they are.
Keep going
- SwordfishThree rows lock one digit into three columns — every other cell of those columns loses it.
- Finned X-WingOne stray place does not kill an X-Wing — it shrinks what you may clear to the fin's box.
Learn the move, not just the digit
In a real game, when a Finned Swordfish is what opens the next cell, the hint in the app names it and shows the cell it unlocks — the same move as step 5 above.