Sudoku techniques · Extreme
Finned Swordfish in Sudoku
A Finned Swordfish is a Swordfish with a fin: one extra candidate on a base line that prevents the ordinary elimination. Candidates still go in the fin's box, on the Swordfish's cover lines and outside the base lines, because those cells are ruled out whether the fin holds the number or not.
Why it works
A fin is one extra cell on a base line that stops a Swordfish from being complete, so the candidate is removed only from cells in the fin's box, outside the base lines, that lie on one of the Swordfish's three crossing lines.
How to spot it
Same discipline as a Finned X-Wing, one line wider.
- Find three base rows whose candidate columns union to four instead of three: the odd one out is your fin.
- The fin must sit in a box that one of the cover columns passes through.
- Expect one or two eliminations, not a sweep.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
9 almost forms a Swordfish in three base rows. If one extra cell were not there, every 9 candidate in those rows would sit in one of only three columns. That one extra cell is the fin.
- 2
Either 9 is in the fin, or the Swordfish is complete. Either way we can remove the candidate 9 from the cells of the fin's box that lie in one of the three columns and outside the three base rows.