Leviathan

Also called Fish of size 7, 7-Fish.

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

What it means

A seven by seven version. It exists, and you will never need it.

The exact rule, for stronger players

The rule, stated exactly: The size-seven basic fish.

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

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

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 Leviathan. In c1, c2, c4, c5, c6, c8, c9 every 2 lies in r1, r2, r4, r5, r6, r7, r8. Each of those columns needs a 2, and between them they use up r1, r2, r4, r5, r6, r7, r8 entirely, so no other cell in those rows can be 2. — In c1, c2, c4, c5, c6, c8, c9 every 2 lies in r1, r2, r4, r5, r6, r7, r8. Each of those columns needs a 2, and between them they use up r1, r2, r4, r5, r6, r7, r8 entirely, so no other cell in those rows can be 2.

Sudoku grid showing Leviathan. In r1, r3, r4, r5, r7, r8, r9 every 3 lies in c3, c4, c5, c6, c7, c8, c9. Each of those rows needs a 3, and between them they use up c3, c4, c5, c6, c7, c8, c9 entirely, so no other cell in those columns can be 3. — In r1, r3, r4, r5, r7, r8, r9 every 3 lies in c3, c4, c5, c6, c7, c8, c9. Each of those rows needs a 3, and between them they use up c3, c4, c5, c6, c7, c8, c9 entirely, so no other cell in those columns can be 3.

Learn these first

WhaleA six by six version.

Related basic fish

SquirmbagA five by five version of the same idea.
SwordfishThe same idea as an X-Wing but with three rows and three columns.
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.
JellyfishThe X-Wing idea stretched to four rows and four columns.

How often does it come up?

Our rough estimate: never required. Its size-two complement is an X-Wing. Included so a reader who meets the name in older material can place it.

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