Squirmbag

Also called Starfish, Fish of size 5, 5-Fish.

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

What it means

A five by five version of the same idea. In a normal grid there is always a smaller pattern that does the same job, so almost nobody looks for this.

The exact rule, for stronger players

The rule, stated exactly: The size-five basic fish. Five base lines confined to five cover lines.

What it removes: Remove d from the five cover lines outside the base lines.

Why it works: The same counting argument at size five.

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 Squirmbag. In r2, r3, r4, r7, r8 every 5 lies in c1, c2, c3, c7, c9. Each of those rows needs a 5, and between them they use up c1, c2, c3, c7, c9 entirely, so no other cell in those columns can be 5. — In r2, r3, r4, r7, r8 every 5 lies in c1, c2, c3, c7, c9. Each of those rows needs a 5, and between them they use up c1, c2, c3, c7, c9 entirely, so no other cell in those columns can be 5.

Sudoku grid showing Squirmbag. In r2, r4, r7, r8, r9 every 1 lies in c2, c4, c5, c8, c9. Each of those rows needs a 1, and between them they use up c2, c4, c5, c8, c9 entirely, so no other cell in those columns can be 1. — In r2, r4, r7, r8, r9 every 1 lies in c2, c4, c5, c8, c9. Each of those rows needs a 1, and between them they use up c2, c4, c5, c8, c9 entirely, so no other cell in those columns can be 1.

Learn these first

JellyfishThe X-Wing idea stretched to four rows and four columns.

Related basic fish

SwordfishThe same idea as an X-Wing but with three rows and three columns.
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.
LeviathanA seven by seven version.

How often does it come up?

Our rough estimate: effectively never required. In a 9x9 grid any size-five fish has a size-four complement in the opposite orientation, so a Jellyfish is always available instead. Documented for completeness and because the name appears in older material.

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