Jellyfish
Short answer: find four columns in which one shape can only go inside the same four rows. Four columns each need that shape and only those four rows can hold it, so the rows are fully committed — the shape leaves every other cell in all four.
What a Jellyfish looks like
X-Wing, Swordfish, Jellyfish: two lines, three lines, four lines, one argument. This is where the family stops being worth hunting for. There is a size-five version and a size-six version, but by then the complementary pattern in the other direction is always smaller and easier to see, so nobody looks for them.
A Jellyfish is drawn here as four columns confined to four rows, which is the same pattern as four rows confined to four columns seen sideways. Both directions are worth checking, and neither is more likely than the other.
How to use a Jellyfish
- Rule out the easy half first. Count how many cells can still hold the shape in total. If it is more than about sixteen there is no Jellyfish worth finding, and if it is very few an X-Wing or Swordfish will already be visible. Jellyfish live in the middle.
- List the rows each column allows. For your chosen shape, note for each column which rows can still hold it. Keep the columns with two, three or four possible rows and discard the rest.
- Find four columns sharing four rows. Pool the rows of four such columns. Exactly four rows in the pool is a Jellyfish; five is nothing. A column using only two of the four rows is perfectly fine.
- Strike the four rows. Remove the shape from every cell of all four rows that is not in one of the four columns. Then check the same shape with rows and columns swapped.
A Jellyfish that is one cell away from working is not wasted: if that stray cell sits in a single box, it is a finned Jellyfish and still eliminates inside that box. Going the other way, if only three columns are involved you have a Swordfish, which is easier to justify and easier to check.
Jellyfish vs Swordfish
Nothing changes but the size, and yet Jellyfish are much harder to trust. With four lines and up to sixteen cells in play it is genuinely easy to miscount, and a single missed candidate outside the pattern makes every elimination invalid. Verify a Jellyfish before you act on it: for each of the four lines, confirm the shape has no home outside the four crossing lines. If you find yourself hunting for a Jellyfish often, it is usually a sign that a colouring chain on the same shape is available and easier.
Find Jellyfish in your own grid
Paste a puzzle and this page will look for 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 there anything bigger than a Jellyfish?
Mathematically yes — five lines is a Squirmbag, six is a Whale — but they are never needed. 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 smaller one is there to be found instead. Jellyfish is the practical end of the family.
How do I check a Jellyfish without making a mistake?
Take the four lines one at a time and ask only one question of each: does this shape have any home outside the four crossing lines? Four yes/no answers is far more reliable than trying to see the whole pattern at once, which is how most false Jellyfish get struck.
Does the shape have to appear in all four crossing lines?
No. Each of the four base lines only has to stay inside the four crossing lines; it can use two of them, or three. Requiring all four everywhere would describe a pattern that almost never occurs.
Is a Jellyfish worth learning?
Only after the rest. It is the rarest technique on this site and the most error-prone, and most puzzles that appear to need one actually fall to a finned Swordfish or a multi-colouring. Learn it to finish the fish family, not because you will use it often.