Jellyfish

Also called Fish of size 4, 4-Fish.

New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.

What it means

The X-Wing idea stretched to four rows and four columns. Rare, and usually there is something simpler to find first.

The exact rule, for stronger players

The rule, stated exactly: For digit d, four base lines each containing d in at most four cells, all confined to four cover lines of the opposite orientation, license elimination of d from the cover lines outside the base lines.

What it removes: Remove d from all cells of the four cover lines outside the four base lines.

Why it works: The size-four case of the same counting argument. Four placements confined to four covers saturate them.

Worked examples

These come from real Proven Sudoku boards. Nobody drew them by hand to make the pattern look neat. Each one is checked first, so the board is one you could really meet, the pattern is really there, and every digit it rules out is really wrong.

Sudoku grid showing Jellyfish. In r3, r4, r7, r8 every 5 lies in c1, c2, c3, c7. Each of those rows needs a 5, and between them they use up c1, c2, c3, c7 entirely, so no other cell in those columns can be 5. — In r3, r4, r7, r8 every 5 lies in c1, c2, c3, c7. Each of those rows needs a 5, and between them they use up c1, c2, c3, c7 entirely, so no other cell in those columns can be 5.

Sudoku grid showing Jellyfish. In r1, r3, r8, r9 every 5 lies in c2, c3, c4, c6. Each of those rows needs a 5, and between them they use up c2, c3, c4, c6 entirely, so no other cell in those columns can be 5. — In r1, r3, r8, r9 every 5 lies in c2, c3, c4, c6. Each of those rows needs a 5, and between them they use up c2, c3, c4, c6 entirely, so no other cell in those columns can be 5.

Learn these first

SwordfishThe same idea as an X-Wing but with three rows and three columns.

Related basic fish

LeviathanA seven by seven version.
SquirmbagA five by five version of the same idea.
WhaleA six by six version.
X-WingA digit is stuck in the same two columns in two different rows, making a rectangle.
Basic Fish (base and cover formulation)Pick some rows where a digit has only a few spots left.

How often does it come up?

Our rough estimate: rare and usually degenerate. In a nine-line grid a size-four fish has a size-five complement, so a Jellyfish always implies a complementary fish in the other orientation.

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