Sudoku·8 min read·May 21, 2026

Sudoku swordfish and jellyfish: the advanced fish patterns

Once you’ve internalized X-wing, the next two techniques in the “fish” family are swordfish (3-row) and jellyfish (4-row). Same core logic, scaled up. They unlock eliminations that X-wing physically can’t see, and they’re what separates Expert from Extreme on most modern sudoku ladders.

Recap: X-wing in one paragraph

A candidate appears in two rows, AT EXACTLY THE SAME TWO COLUMNS in each row. The logic forces those two columns to contain that candidate within those two rows, so you can eliminate the candidate from those columns in every OTHER row.

If this doesn’t feel automatic yet, work through the hard-sudoku guide first. Swordfish only clicks once X-wing is muscle memory.

Swordfish, the 3-row fish

The pattern:A candidate (let’s say 4) appears in THREE rows, and across those three rows the 4 only ever shows up in the same THREE columns. The columns don’t need to match cell-for-cell, each row needs the 4 only in some subset of the three columns.

The conclusion: Same as X-wing extended. Within those three rows the 4s must go into those three columns (some permutation). So 4 can be eliminated from those columns in every OTHER row.

Concrete example:

Row 2: candidate 4 appears at columns 1, 5, 7 (only these three).
Row 5: candidate 4 appears at columns 1, 7 (subset of {1, 5, 7}).
Row 9: candidate 4 appears at columns 5, 7 (subset of {1, 5, 7}).

Together these three rows contain THREE 4s total, distributed into THREE columns (1, 5, 7) in some permutation. No other row in columns 1, 5, or 7 can contain a 4. Erase 4 from those columns in rows 1, 3, 4, 6, 7, 8.

The shape of a swordfish (in pencil marks)

In practice swordfish doesn’t look like a fish. It looks like “this candidate is unusually constrained.” The scan to spot one:

Pick a candidate value (say, 4).
For each row, count how many cells still have 4 as a candidate. If a row has 4 in MORE than three cells, it can’t be part of a swordfish.
Find three rows where 4 appears in 2-3 cells each, AND all those cells together span only three columns.
That’s your swordfish. Eliminate 4 from those three columns in every other row.

Swordfish works equally well with columns: candidate appearing in only 3 columns total, all within the same 3 rows. Same logic transposed.

Jellyfish, the 4-row fish

Same idea, one size up. A candidate appears in four rows, only across four columns. Within those four rows the candidate must end up distributed among those four columns. Eliminations cascade to every other row in those four columns.

Jellyfish is rare. On most Expert puzzles you can finish with naked singles, hidden singles, naked pairs, pointing pairs, and X-wing. Jellyfish typically only comes up on Extreme puzzles where every easier technique has already been applied and a few cells remain.

Spotting it:same scan as swordfish, extended to four rows. The candidate needs to appear in four rows where each row has the candidate in 2-4 cells, and the union of those cells’ columns is exactly four.

Why no quintfish, sextfish, etc.?

The fish family generalizes infinitely, but useful variants stop at jellyfish for a deep mathematical reason: a fish of size N implies a fish of size 9-N when you flip rows and columns. So a 5-fish (quintfish?) is mathematically equivalent to a 4-fish (jellyfish) from the opposite axis. A 6-fish equals a 3-fish (swordfish). And so on.

In practice, after jellyfish, the next technique that earns its own name is the XY-wing, different family entirely (chain logic instead of fish logic).

When to actually look for these

These techniques are expensive to scan for. Order of operations on a hard puzzle:

Naked singles → exhaust.
Hidden singles → exhaust.
Naked pairs / triples → exhaust.
Pointing pairs + box-line reduction → exhaust.
X-wing → if nothing else moves.
Swordfish → only if X-wing didn’t fire AND you’re still stuck.
Jellyfish → after swordfish.
XY-wing and beyond → for the absolute hardest puzzles.

Most Extreme puzzles on Melio Sudoku resolve at step 6 or 7. Truly extreme outliers need step 8.

The technique nobody talks about: walking away

Counterintuitive truth: on hard puzzles, walking away for ten minutes and coming back finds the next move roughly as often as scanning harder does. Visual fatigue makes you scan the same area twice without seeing it. Fresh eyes catch patterns you’d been staring through.

Build the break into your routine. After 15 minutes of hard stuck, leave the puzzle. Come back. The breakthrough comes within 60 seconds about half the time.

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Sudoku skyscraper: the cousin of X-wing for short-strong patterns

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Browse all guides →

Recap: X-wing in one paragraph

If this doesn’t feel automatic yet, work through the hard-sudoku guide first. Swordfish only clicks once X-wing is muscle memory.

Swordfish, the 3-row fish

The conclusion: Same as X-wing extended. Within those three rows the 4s must go into those three columns (some permutation). So 4 can be eliminated from those columns in every OTHER row.

Concrete example:

Row 2: candidate 4 appears at columns 1, 5, 7 (only these three).
Row 5: candidate 4 appears at columns 1, 7 (subset of {1, 5, 7}).
Row 9: candidate 4 appears at columns 5, 7 (subset of {1, 5, 7}).

The shape of a swordfish (in pencil marks)

In practice swordfish doesn’t look like a fish. It looks like “this candidate is unusually constrained.” The scan to spot one:

Pick a candidate value (say, 4).
For each row, count how many cells still have 4 as a candidate. If a row has 4 in MORE than three cells, it can’t be part of a swordfish.
Find three rows where 4 appears in 2-3 cells each, AND all those cells together span only three columns.
That’s your swordfish. Eliminate 4 from those three columns in every other row.

Swordfish works equally well with columns: candidate appearing in only 3 columns total, all within the same 3 rows. Same logic transposed.

Jellyfish, the 4-row fish

Why no quintfish, sextfish, etc.?

In practice, after jellyfish, the next technique that earns its own name is the XY-wing, different family entirely (chain logic instead of fish logic).

When to actually look for these

These techniques are expensive to scan for. Order of operations on a hard puzzle:

Naked singles → exhaust.
Hidden singles → exhaust.
Naked pairs / triples → exhaust.
Pointing pairs + box-line reduction → exhaust.
X-wing → if nothing else moves.
Swordfish → only if X-wing didn’t fire AND you’re still stuck.
Jellyfish → after swordfish.
XY-wing and beyond → for the absolute hardest puzzles.

Most Extreme puzzles on Melio Sudoku resolve at step 6 or 7. Truly extreme outliers need step 8.

The technique nobody talks about: walking away

Build the break into your routine. After 15 minutes of hard stuck, leave the puzzle. Come back. The breakthrough comes within 60 seconds about half the time.