Techniques › Jellyfish
Jellyfish
The four-row fish. Valid, beautiful, and almost never necessary.
Four rows, four columns, same argument as the X-Wing and the Swordfish.
A Jellyfish is almost always accompanied by a simpler deduction elsewhere on the board, so a solver working cheapest-first will rarely need one — published incidence is around ten puzzles in 141,672. It is included here for completeness, and because the progression from two rows to four is the clearest illustration of what a fish actually is.
Beyond four there is no point: a five-row fish in a 9×9 grid is always mirrored by a four-row fish in the other direction.
A worked example
Taken from a real puzzle in our bank, at the point where this was the cheapest move available. Pattern candidates are marked in amber; anything the step eliminates is struck through in red.
| 6 | 12 | 125 | 9 | 3 | 8 | 17 | 157 | 4 |
| 3 | 9 | 8 | 7 | 14 | 145 | 6 | 15 | 2 |
| 7 | 4 | 15 | 6 | 12 | 125 | 8 | 9 | 3 |
| 8 | 7 | 12 | 3 | 6 | 12 | 9 | 4 | 5 |
| 14 | 126 | 46 | 5 | 127 | 9 | 3 | 8 | 17 |
| 5 | 3 | 9 | 8 | 147 | 14 | 2 | 17 | 6 |
| 9 | 8 | 3 | 4 | 5 | 6 | 17 | 2 | 17 |
| 2 | 5 | 7 | 1 | 9 | 3 | 4 | 6 | 8 |
| 14 | 16 | 46 | 2 | 8 | 7 | 5 | 3 | 9 |
- NudgeDigit 1 lines up in a rectangular pattern across several lines. Nothing outside it can be 1.
- WhereJellyfish in row 2, row 3, row 4, row 6, column 3, column 5, column 6, column 8: r2c5, r2c6, r2c8, r3c3, r3c5, r3c6, r4c3, r4c6, r6c5, r6c6, r6c8
- Do itRemove r1c3 ≠ 1, r5c5 ≠ 1, r1c8 ≠ 1
How to spot it
Four rows whose homes for one digit span exactly four columns. If you find one before finding something simpler, check again — there is usually something simpler.
Where you will meet it
Jellyfish sits at tier 5 of our ladder, which means it first becomes necessary at Expert. Puzzles below that band never require it, though it may still appear as a shortcut.