Finned Swordfish

A Finned Swordfish is a Swordfish that almost works. Three rows would lock a digit into three columns, except one row carries one extra candidate for that digit. That extra candidate is the fin, and it does not kill the pattern — it only shrinks the list of cells you can clear.

What the pattern is

A clean Swordfish on digit d uses three base lines (three rows, say). In each base line every d sits in the same three columns, the cover lines. Because the three rows need three different columns between them, d is used up in those columns and can be removed from them everywhere else.

A Finned Swordfish breaks that neatness in one place. One base row has an additional d outside the cover columns, and that extra candidate sits in a box that also holds one of the pattern cells of the same row. Now there are two possibilities:

• The fin is false. The Swordfish is real, and all its normal eliminations hold.
• The fin is true. Then that cell is d, so d leaves the rest of its box.

Anything both cases agree on is safe to remove. That means only cover-line cells inside the fin's box.

When to look for it

Look after a plain fish search fails. If you find three rows where d is nearly confined to three columns and only one stray candidate spoils it, check whether that stray shares a box with a pattern cell in its own row. If it does, you have a Finned Swordfish. When the pattern cell in the fin's box carries no d at all, the shape is called sashimi and the same conclusion still holds.

How to apply it, step by step

1. Pick a digit and list, row by row, the columns where it can still go.
2. Find three rows whose candidates fall into three columns plus at most one intruder.
3. Check the intruder: it must lie in a box that also contains one of that row's pattern cells.
4. Take the cover columns and keep only the cells inside the fin's box.
5. Remove the digit from those cells, ignoring cells in the base rows.
6. Repeat with columns as base lines and rows as covers.

A worked example

Digit 4, base rows r2, r5, r8, cover columns c1, c5, c9. Row 8 carries one intruder at r8c2, inside box 7 with the pattern cell r8c1.

digit 4 only:  X = 4 possible,  F = fin,  . = 4 already gone

      c1 c2 c3 | c4 c5 c6 | c7 c8 c9
 r2    X  .  . |  .  X  . |  .  .  .
 r5    .  .  . |  .  X  . |  .  .  X
 r8    X  F  . |  .  .  . |  .  .  X

 r7    X  .  . |  .  .  . |  .  .  .   -> 4 removed
 r9    X  .  . |  .  .  . |  .  .  .   -> 4 removed

If r8c2 is not 4, rows 2, 5 and 8 form a true Swordfish and column 1 loses its other 4s. If r8c2 is 4, box 7 loses its other 4s. Both roads remove 4 from r7c1 and r9c1. Nothing in columns 5 or 9 can be touched, because those cells do not see the fin.

Common mistakes

• Taking the full Swordfish eliminations anyway. Without the fin restriction the deduction is simply wrong.
• Accepting a fin that sits in a box with no pattern cell from its own row. Then the two cases share no conclusion.
• Allowing two stray candidates in different boxes. One box only.
• Forgetting the transposed search. Half of all finned fish appear only when you use columns as base lines.

Where you meet it

Finned Swordfish belongs to the extreme tier. You will rarely need it before Master, and it shows up regularly in Master puzzles where a clean fish is one candidate short of working. Learn the finless Swordfish first; the fin is a small extra rule on top of a pattern you already know.