Jellyfish Sudoku Technique: Use Cases & Examples

Jellyfish is a single-candidate elimination strategy that builds on X-wing and swordfish by using a four row and four column structure, allowing you to eliminate candidates outside the pattern in hard Sudoku puzzles.

If you’re already familiar with X-wing and swordfish patterns, learning the jellyfish technique will help you solve more advanced puzzles when you play Sudoku online.

How to Find a Jellyfish Pattern

A jellyfish is part of the fishy patterns family in Sudoku. Like other fishy techniques, jellyfish doesn't tell you where the digit must go, but it lets you confidently eliminate it from incorrect cells, narrowing down your options.

This Sudoku strategy is just like the swordfish, but expanded to include four rows and columns instead of three.

To spot a jellyfish

1. Pick a single candidate to focus on.
2. Check if it appears in two to four cells across four different rows. These rows are called your base sets.
3. See if those candidate cells are all in the same four columns. These are your cover sets.

Based on Sudoku’s uniqueness rule, each row and column can contain the candidate only once. Since the candidate appears in the four base rows only within the same four columns, it must occupy one cell in each row and column. So, you can eliminate that candidate from any other cells in those columns outside the base rows.

Here’s what a perfect jellyfish (though rare) looks like:

• Candidate X appears in rows 1, 3, 6, and 8, which are your base sets.
• In those rows, it only shows up in columns B, D, E, and H, which are your cover sets.
• This 4x4 setup locks the candidate into those 16 cells, so you can confidently remove X from any other cells in columns B, D, E, and H that aren’t part of the jellyfish (B4, D7, E2, H9).

You can also flip this approach and scan columns first. If the candidate appears in four columns and all the cells line up in the same four rows, then the columns are the base sets and the rows are the cover sets. The logic and elimination rule work the same way.

Jellyfish Sudoku Examples

Jellyfish can be oriented horizontally or vertically, just like X-wings and swordfish patterns. The orientation refers to whether the base sets are rows (horizontal orientation) or columns (vertical orientation). As long as you’re using pencil marks, you’ll be able to spot a jellyfish, and these examples walk you through how to do it.

Jellyfish Pattern with Horizontal Orientation (Example 1)

To identify a jellyfish, look for these criteria. With practice, you can simplify the technique into two steps like the examples below.

1. Check to see if the candidate appears in two to four cells across four different rows. For example, 5 appears in four different rows (1, 4, 6, and 7). The four rows are called base sets.
2. Confirm that all of those candidates lie within the same four columns. In this example, candidate 5 appears in three cells in each row (C1, D1, G1; C4 D4, I4; C6, G6, I6; D7, G7, and I7) and four aligned columns (C, D, G, and I). The columns are called cover sets.
3. Eliminate the candidate from any cells in the cover sets that are not part of the base sets. Since the base sets are rows, you can eliminate candidate 5 from any cells in columns C, D, G, and I that aren’t in rows 1, 4, 6, or 7.

This works because each digit can appear only once per row, so if a candidate is limited to specific columns within each row, it must occupy one of those positions, meaning any other instance of that candidate in those columns, outside those rows, can be safely removed.

Jellyfish Pattern with Horizontal Orientation (Example 2)

When a single candidate is limited to four rows and within the same four columns, you have a horizontal jellyfish pattern. Jellyfish has two simple steps to identify it and eliminate candidates:

1. Identify a single candidate that appears two to four times in four rows and in the same four columns. For example, candidate 4 appears only three or four times in rows 2, 4, 6, and 9. When it appears, the candidate aligns with just four columns: A, D, E, and H. The rows are the base sets for the jellyfish pattern.
2. Eliminate the candidate identified in the base sets (rows) from the cover sets (columns). Candidate 4 is limited to only three or four cells in rows 2, 4, 6, and 9, so it must occupy one cell in each of those rows and columns. That means you can eliminate any 4 that appears in the cover sets but not in the base sets (A1, A8, D5, E3, and H7). However, you cannot eliminate 4s that appear outside the jellyfish pattern (B3 and F1).

Jellyfish Pattern with Vertical Orientation

A jellyfish has a vertical orientation if the base sets are columns. To find one, just look at columns as the base sets instead of rows, and follow these steps:

1. Identify a single candidate that appears only two to four times in four columns and no more than four rows. For example, candidate 7 appears just three or four times in columns A, C, F, and I, creating the base sets. And candidate 7 aligns with only four rows (1, 3, 5, and 8) within those four columns.
2. Eliminate the candidate identified in the base sets (columns) from the cover sets (rows). Because candidate 7 is limited to only three or four cells in columns A, C, F, and I, it must be the answer in those cells for rows 1, 3, 5, and 8. Those rows create the cover sets, allowing you to eliminate any 7 that appears in the cover sets but not in the base sets (B8, E5, G1, and H3). However, 7s that appear outside the cover and base sets of the jellyfish pattern cannot be eliminated (E2 and G9).

If you find an X-wing or swordfish, check if you can extend it to a jellyfish so that you can solve more challenging Sudoku puzzles. This jellyfish solving technique is perfect for hard or expert puzzles to help you eliminate candidates and solve puzzles you’re stuck on.