- In the highlighted lines, 5 fits only in the highlighted cells.
- Whichever cells take 5, each crossing line gets one of them.
- So 5 can go nowhere else in the lines that cross them.
Finned Swordfish in sudoku
A Finned Swordfish is a Swordfish in which one of the three rows has extra cells for the number outside the three columns, all in one box. Those cells are the fin. Only the cells of the Swordfish columns that lie in the box of the fin lose the number.
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How to spot it
Look for a Swordfish that almost works: three rows whose cells for a number fall into three columns, apart from one or two cells in a single box. Then check that box for notes of the number in the Swordfish columns, outside the three rows.
Why it works
Either a fin cell holds the number, or none does and the Swordfish is a real one. A cell in the box of the fin and in a Swordfish column loses the number either way.
On a real puzzle
This is Finned Swordfish on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
No level of the game needs Finned Swordfish, so this practice puzzle is harder than Extreme, its hardest level. Fill in what you can, then look for the pattern, or ask for the hint: each game has one.