Jellyfish
A digit confined to four columns across four rows clears them.
A jellyfish is a single-digit elimination pattern — the four-line member of the fish family that begins with the X-Wing and grows through the Swordfish. When one candidate is confined to the same four columns across four different rows, those four rows collectively book all four of those columns, so the digit can be erased from every other cell sitting in them. The jellyfish sudoku technique is the natural next step once smaller fish stop appearing, though it surfaces far less often than its two- and three-line cousins.
Worked example
On this grid, candidate 1 across rows 1 to 4 hides in just four columns: R1C1/R1C2 in row 1, R2C2/R2C3 in row 2, R3C3/R3C4 in row 3, and R4C4/R4C1 in row 4. Four rows, four columns — columns 1, 2, 3 and 4 must each draw their single 1 from one of these four rows. That leaves no cell of columns 1-4 in any other row free to hold a 1, so R5C1 loses its 1 candidate, along with every other cell in those columns outside rows 1-4.
When to use it
Reach for a jellyfish only after you have ruled out X-Wings and Swordfish on the digit — a smaller fish or a simpler move almost always bites first. Pick one candidate, then list, row by row, the columns it can still occupy. Scan for four rows whose candidate columns, pooled together, number exactly four; those four columns are where the eliminations fall.
How to use it
- Find four rows. Pick a digit and find four rows where it can only go inside the same four columns (each row uses two to four of them).
- See the lock. Across those four rows the digit occupies four columns, one per row and per column, so those four columns are spoken for.
- Eliminate down the columns. Remove the digit from every other cell in those four columns. The same pattern works with rows and columns swapped.
Frequently asked questions
What is a Jellyfish in Sudoku?
A jellyfish is a single-digit fish pattern that spans four rows and four columns. If a candidate in four rows is restricted to the same four columns, those rows claim all four columns between them, so the digit is removed from those columns in every other row. In the worked example, digit 1 across rows 1-4 is trapped in columns 1-4, which clears the 1 from R5C1.
How do I spot a Jellyfish?
Choose one digit and write out, row by row, which columns can still hold it. Look for four rows whose candidate columns together add up to no more than four distinct columns — that is the fish. On this board, rows 1-4 each keep digit 1 inside columns 1-4, so the four rows and four columns lock together into a jellyfish.
How is a Jellyfish related to the X-Wing and Swordfish?
They are the same single-digit fish logic at different sizes: an X-Wing spans two lines, a Swordfish three, and a jellyfish four. The four base rows do not each need the digit in all four columns — they only have to use no more than four columns between them, as R1-R4 do here with columns 1-4. Because it is so large, a jellyfish is rare, and a simpler move usually resolves the digit first.