Swordfish

Also called Fish of size 3, 3-Fish.

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

What it means

The same idea as an X-Wing but with three rows and three columns. The digit in those three rows can only sit in three columns, so it fills all three and leaves those columns.

The exact rule, for stronger players

The rule, stated exactly: For digit d, if three rows each contain d in at most three cells, and those cells lie within the same three columns, then d may be eliminated from those columns outside the three rows. Roles may be exchanged.

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

Why it works: Three base lines each place d once, giving three placements. Every candidate position lies in three cover lines, which can absorb at most three placements. The count is exact, so the covers are saturated and every other cell in them is excluded.

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 Swordfish. In r6, r7, r9 every 6 lies in c1, c2, c8. Each of those rows needs a 6, and between them they use up c1, c2, c8 entirely, so no other cell in those columns can be 6. — In r6, r7, r9 every 6 lies in c1, c2, c8. Each of those rows needs a 6, and between them they use up c1, c2, c8 entirely, so no other cell in those columns can be 6.

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

Learn these first

X-WingA digit is stuck in the same two columns in two different rows, making a rectangle.

Related basic fish

WhaleA six by six version.
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.
SquirmbagA five by five version of the same idea.

How often does it come up?

Our rough estimate: uncommon. Many claimed Swordfish are degenerate, meaning a smaller pattern already covers the same eliminations.

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