Fish
Jellyfish
A jellyfish is the next size up from a swordfish: four rows in which a digit has two, three or four possible cells, with every one of those cells inside the same four columns. The four rows have to place the digit four times and have only those four columns to do it in, so the digit leaves the rest of every column.
- Level
- Advanced
The pattern
What it does.
The family runs X-Wing, swordfish, jellyfish: two rows and two columns, three and three, four and four. The argument never changes, only the number of lines it spans.
Four rows that between them use four columns is all it takes. The rows can hold two, three or four homes each, and the pattern still works, because the total is what confines the digit.
A jellyfish clears a digit from four columns at once, which is why it often finishes a puzzle that has resisted everything smaller. It's also correspondingly rare, and it's usually found by following a digit that has already survived a failed X-Wing and swordfish scan.
Example
A worked example.
Take one digit
Scan the whole grid for a single digit and note, for every row, which cells can still take it. Keep the rows with four homes or fewer.
Gather four rows
Take four of those rows and collect the columns their homes fall in. If the four rows together use only four columns, the digit is confined to a four-by-four block of the grid.
Confirm the confinement
Every home must be inside those four columns. One candidate in a fifth column means the digit escapes the block and there's no jellyfish.
Eliminate down the columns
Remove the digit from every other cell in those four columns. The four rows must still place it, and inside the block is the only place left.
Spotting it
How to spot it.
- Chase whichever digit has the fewest homes per row across the grid. A digit that's close to placed is the one worth the scan.
- Don't stop at swordfish. Four rows that each hold a digit two or three times is the usual shape, and it's easy to walk past.
- The transpose works identically: four columns whose homes share four rows clear the digit from the rest of those rows.
- Expect it in the hardest puzzles only. If pairs, triples and the smaller fish are still producing moves, use them first.
Where it stops working
The eliminations are wide but conditional: the confinement has to be exact, and one extra candidate anywhere in the four rows voids the whole pattern. As with every fish, a candidate list that's missing a digit will make the block look tighter than it is, which is the one way this technique removes something true.
Keep going
Related topics.
New to sudoku? The rules take two minutes to read, and the first-puzzle walkthrough covers the basics that come before any of these.
Beyond the lessons.
The game's lessons stop at X-Wing, so there's no Jellyfish lesson. The eight it does teach will get you through almost any puzzle; this page is here for when you run into the pattern anyway.