Jellyfish Sudoku Technique

Updated 11 August 2026

A Jellyfish is the four-line version of the fish pattern behind the X-Wing and the Swordfish: a digit confined to the same four columns across four rows can be erased from every other cell in those four columns.

When a Jellyfish applies

A Jellyfish extends the Swordfish’s relaxed coverage rule one size further. Four rows are chosen, and the digit’s candidate cells across those four rows, taken together, must span exactly four columns. No single row needs candidates in all four columns — two or three is enough per row — as long as the combined span across the four rows lands on exactly four, and no row adds a fifth.

The scanning cost grows quickly with size. An X-Wing compares two rows, a Swordfish compares three, and a Jellyfish compares four, so the number of row combinations to check climbs fast even on a nine-row grid. That is why Jellyfish patterns are rare in practice and are usually left to a solving engine rather than searched for by hand, even though the underlying logic is no different from the smaller fish.

The same pattern also runs on the column axis: four columns whose candidate cells for a digit span exactly four rows trap the digit just as tightly, eliminating it from those four rows elsewhere.

Worked example

Across rows 1, 5, 6 and 9, the only cells that can still hold 8 are r1c3, r1c6, r5c3, r5c5, r5c6, r6c1, r6c3, r6c5, r9c3, r9c5 and r9c6. Collect the columns those eleven cells use: 1, 3, 5 and 6 — exactly four distinct columns spread across exactly four rows. That is a Jellyfish on 8, even though not every row reaches all four columns; row 1 only reaches columns 3 and 6, for example, while row 6 reaches all four.

Because rows 1, 5, 6 and 9 between them must place 8 in columns 1, 3, 5 and 6, no other row can hold an 8 in any of those four columns. r2c1 was carrying 3, 6, 7 and 8 before the deduction and loses the 8, leaving 3, 6 and 7. The same reasoning also clears 8 from r8c1, r2c3, r2c6 and r8c6 on this board, so the Jellyfish removes five stray candidates in one pass even though it needed eleven cells to prove it.

How to spot a Jellyfish

1. Pick a digit and list its candidate cells per rowFor one digit, write down which columns still hold it in each row. Rows where the digit has five or more candidate cells are not useful.
2. Look for four rows spanning four columnsFind four rows whose candidate columns for the digit, combined, total exactly four distinct columns. Each row can use two, three or four of them.
3. Reject a fifth columnIf any of the four rows has a candidate in a column outside the shared set of four, the pattern does not hold. Every candidate cell in the four rows must sit in one of the four columns.
4. Erase the digit from the rest of the four columnsRemove the digit from every other cell in the four columns, outside the four base rows, then repeat the search on the column axis.

Common mistakes

• Requiring every row to use all four columns. A row can use as few as two of the four, as long as the combined span across all four rows stays at four.
• Missing a fifth column hiding in one of the four rows. A single stray candidate cell outside the shared columns invalidates the whole pattern.
• Trying to spot a Jellyfish by eye on a busy board. With four rows and up to sixteen candidate cells to track, it is far easier to verify with pencil marks written down than to hold in your head.
• Assuming a Jellyfish is common. Most puzzles that need a fish pattern at all resolve with an X-Wing or a Swordfish; a true Jellyfish is a sign of a genuinely hard grid.