GridWits
Extreme tier Technique 32 of 46 Used by Smart Hint

Finned Swordfish

An almost Swordfish whose extra fin candidates are confined to one box — eliminations survive only where a cover line crosses that box.

How it works

  1. 1Rows 1, 4 and 7 almost form a Swordfish for digit 8 across columns A, D and G.
  2. 2H7 is the extra fin. It shares the bottom-right box with the row-7 fish corner at G7.
  3. 3If H7 is 8, the fin's box loses its other 8s. If H7 is not 8, the ordinary Swordfish holds and columns A, D and G lose their outside 8s.
  4. 4G8 is caught in both cases: it lies on cover column G outside the base rows and inside the fin's box, so its 8 is eliminated.
  5. 5Unlike a Finned X-Wing, this proof has three base rows and three cover columns; the fin restriction is otherwise identical.

Play a Sudoku that needs this technique →

A 1 B 2 C 3 D 4 E 5 F 6 G 7 H 8 I 9 2 8 5 8 1 8 4 8 6 8 3 8 7 8 8 9 8 8 8 8 8 8 8 8
  • First colour in the chain
  • Cells that establish the pattern
  • Where the conclusion lands
  • Candidate ruled out
Next in the ladder Forcing Nets →

Forcing Chains generalized into a branching tree instead of a single line — every branch from an assumption is followed, and any conclusion every branch agrees on holds.