Whale

Also called Fish of size 6, 6-Fish.

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

What it means

A six by six version. Listed because people search for the name, not because you need it.

The exact rule, for stronger players

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

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

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

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 Whale. In r2, r3, r5, r6, r7, r9 every 1 lies in c1, c4, c5, c6, c7, c8. Each of those rows needs a 1, and between them they use up c1, c4, c5, c6, c7, c8 entirely, so no other cell in those columns can be 1. — In r2, r3, r5, r6, r7, r9 every 1 lies in c1, c4, c5, c6, c7, c8. Each of those rows needs a 1, and between them they use up c1, c4, c5, c6, c7, c8 entirely, so no other cell in those columns can be 1.

Sudoku grid showing Whale. In r1, r2, r3, r5, r8, r9 every 5 lies in c1, c2, c5, c7, c8, c9. Each of those rows needs a 5, and between them they use up c1, c2, c5, c7, c8, c9 entirely, so no other cell in those columns can be 5. — In r1, r2, r3, r5, r8, r9 every 5 lies in c1, c2, c5, c7, c8, c9. Each of those rows needs a 5, and between them they use up c1, c2, c5, c7, c8, c9 entirely, so no other cell in those columns can be 5.

Learn these first

SquirmbagA five by five version of the same idea.

Related basic fish

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

How often does it come up?

Our rough estimate: never required in a 9x9 grid. Its size-three complement is a Swordfish, which is always found first. Included for naming completeness only.

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