Swordfish vs Jellyfish: Extending the Sudoku Fish Pattern

The Foundation: Recapping the X-Wing (2x2 Fish)

Before tackling larger fish, you must be fluent with the X-Wing. An X-Wing occurs when a candidate appears exactly twice in two rows, and those four cells line up in exactly two columns. This pattern proves the candidate must occupy the two diagonal corners of the imaginary rectangle, allowing you to eliminate it from all other cells in those two columns. Think of the Swordfish and Jellyfish as multi-row, multi-column extensions of this same 'either/or' logic. If you struggle to spot X-Wings, practicing them is the essential first step before moving to larger patterns.

The Swordfish: A 3x3 Fish Pattern

A Swordfish pattern is defined by a single candidate. You can search for it by rows or by columns. When searching by rows, you look for three rows where the candidate appears in 2 or 3 cells per row. Crucially, all of these cells must be confined to only three columns. The candidate cannot appear elsewhere in these three rows. This alignment creates a powerful constraint: the candidate must be placed in three of the nine defining cells, one in each row and one in each column. Therefore, you can eliminate this candidate from all other cells in the three defining columns. The same logic works in reverse: find three columns with the candidate in 2 or 3 cells each, all lying in exactly three rows, and eliminate the candidate from the rest of those rows. For a deeper dive, see our Swordfish Strategy guide.

• Start your search by looking for a candidate that appears frequently (4-6 times) in a band of three rows.
• Use pencil marks! A Swordfish is nearly impossible to spot without full candidate notation.
• If you find a potential 3x3 pattern but one row has the candidate in 4 columns, it is not a valid Swordfish.

The Jellyfish: A 4x4 Fish Pattern

The Jellyfish is the natural extension of the Swordfish, applying the same logic across four units instead of three. To find a Jellyfish by rows, look for four rows where your target candidate appears in 2, 3, or 4 cells per row. All these cells must be contained within only four columns. Because the candidate must occupy one cell per row and one per column within this 4x4 grid, it can be safely removed from all other cells in those four defining columns. A column-based Jellyfish uses four columns with cells confined to four rows, allowing eliminations in those rows. Jellyfish are rare in all but the most extreme puzzles, but recognizing the pattern can crack a seemingly impossible grid. Explore more examples in our Jellyfish Strategy article.

• Jellyfish are uncommon. Before spending time looking for one, exhaust all other advanced techniques like Swordfish and Y-Wing.
• The pattern does not require the candidate to appear in every cell of the 4x4 grid. It only needs to be confined to those four columns across the four rows.
• Scan for candidates that appear 6-8 times across the puzzle; they are the most likely to form large fish patterns.

Side-by-Side Comparison

While Swordfish and Jellyfish operate on identical principles, their practical differences are significant. A Swordfish uses a 3-row, 3-column framework and is a relatively common technique in hard puzzles. Our hint engine shows it appears in about 15% of hard-rated puzzles. A Jellyfish uses a 4-row, 4-column framework and is much rarer, appearing in perhaps 2% of extreme 'Diabolical' or 'Evil' puzzles. In terms of spotting difficulty, a Jellyfish is not inherently more complex to understand, but its larger size makes it visually harder to identify in a grid full of pencil marks. Both are pure 'fish' patterns, distinct from 'finned' or 'sashimi' variants which involve exceptions.

Worked Examples

**Swordfish Example (by Row):** Imagine the candidate 7 appears in rows 2, 5, and 8. In row 2, it is in columns 3 and 6. In row 5, it is in columns 1 and 6. In row 8, it is in columns 1 and 3. All cells are in just three columns: 1, 3, and 6. This is a perfect Swordfish. You can eliminate candidate 7 from all other cells in columns 1, 3, and 6.

**Jellyfish Example (by Column):** Suppose the candidate 5 appears in columns 2, 4, 7, and 9. In these columns, the candidate's cells are found only within rows 1, 3, 6, and 8. This defines a Jellyfish by column. You can eliminate candidate 5 from every other cell in rows 1, 3, 6, and 8.

Relation to Finned and Sashimi Fish

The Swordfish and Jellyfish described here are 'classic' or 'pure' fish. Real puzzles often contain imperfect versions. A 'finned' fish has the core pattern plus one extra candidate (the 'fin') in one of the defining rows or columns, but outside the defining columns or rows. The elimination rule changes slightly, often allowing removal of the candidate from cells that see both the fin and the fish's 'body'. A 'sashimi' fish is a more extreme case where one of the base rows or columns contains the candidate only in the fin cell, breaking the pure pattern. These are advanced topics, but knowing they exist explains why a perfect 3x3 or 4x4 pattern is not always required for a valid elimination.