Finned Jellyfish
Short answer: a Jellyfish in which one line has a cell outside the four crossing lines. If that fin sits in one box, then either the Jellyfish is real or the fin holds the shape — and both put the shape in that box, so it goes from the covered cells there.
What a finned Jellyfish looks like
This is the largest pattern on the site and the last one worth knowing. Four lines, up to sixteen cells, and one of them out of place. Nobody spots this by looking; it is found by writing down where a shape can go and noticing that four lines nearly agree.
Be honest about when this is worth reaching for. A finned Jellyfish is the move you make when a full pass of everything else has produced nothing at all — which does happen, and is exactly why the technique exists.
How to use a finned Jellyfish
- Check the shape is worth chasing. Count the cells that can still hold your shape. Fewer than about eight and a smaller fish will already be there; more than about eighteen and there is nothing to find. In between is where a four-line fish lives.
- Write the lines down. For each row, list the columns that can still hold the shape. Doing this on paper rather than in your head is not optional at this size — this is where every mistaken Jellyfish comes from.
- Find four lines that pool to five. Four rows whose columns pool to exactly four is a clean Jellyfish. Pooling to five means one intruder: that is the fin, and it must sit in a single box.
- Strike the intersection only. Remove the shape from the cells that are in one of the four crossing lines AND in the fin’s box, and are not part of the pattern. Usually one or two cells — which is often enough.
Before trusting it, look for the same shape in the other direction: a five-line pattern one way is always a four-line pattern the other way, so a Jellyfish that seems to need five lines has a smaller reading you have missed. And if the fin is in the pattern's own box rather than outside the crossing lines, you have a clean Jellyfish and can strike all four lines.
Finned Jellyfish vs multi-colouring
These two are rivals: both are last-resort techniques on a position where everything else has failed, and they often live in the same grid. Multi-colouring is usually the better bet, because building conjugate chains is mechanical and self-checking, while a four-line fish demands that you correctly enumerate sixteen cells before you strike anything. Reach for the fish when the shape has too few conjugate pairs for chains to get anywhere.
Find Finned Jellyfish in your own grid
Paste a puzzle and this page will look for Finned Jellyfish in it — the same pattern shown above, in your grid instead of ours. Nothing is uploaded; it runs in your browser.
Frequently asked questions
Is this ever really necessary?
Occasionally, and only in puzzles built to be hard. On an ordinary "expert" grid from a newspaper you will never need it. It matters for the handful of positions where every wing, chain and colouring comes up empty and there is still no guess to be avoided.
How do I avoid miscounting?
Ask each line one yes/no question — does the shape have a home outside the four crossing lines? — and write the answers down. Four answers is checkable. Trying to hold a sixteen-cell pattern in your head is not, and that is where false eliminations come from.
Can it have two fins?
Yes, provided both are in the same box. The case split only needs "if the fish fails, this box holds the shape", and two cells in one box still deliver that.
Why not go bigger than four lines?
Because there is no need. In a 9×9 grid a fish of size five in one direction always implies a fish of size four in the other, so the four-line version is always available and always easier to check. Four is where the family ends.