What is a Jellyfish?
Jellyfish is a “4‑fish” pattern: a technique that uses four rows (or four columns) where a single candidate digit is confined to exactly four columns (or rows). When you find such a configuration, you can eliminate that digit from any other cells in those four columns (or rows) that are outside the four chosen rows (or columns). It’s a step up from the simpler X‑wing (2‑fish) and Swordfish (3‑fish), and it often appears when a puzzle resists more basic methods.
When to look for it
Look for a Jellyfish after you’ve exhausted single-candidate fills, naked pairs/triples, and X‑wings and Swordfish checks. It’s most useful in medium–hard puzzles where one digit is stubborn across many rows or columns. Don’t waste time scanning every digit early in the solve; instead, try it when you notice a candidate repeatedly appearing in similar columns across four different rows (or vice versa).
How it works (plain language)
1) Pick a digit to test (for example, 5).
2) Find four rows that contain candidate 5 in only a subset of columns. If across those four rows the candidate 5 appears in no more than four distinct columns, you may have a Jellyfish.
3) Confirm that, for each of those four columns, every 5-candidate in that column is confined to those four rows (i.e., there is no 5 candidate in that column outside those rows).
4) If confirmed, 5 can be eliminated from any other cell that lies in any of those four columns but not in the four rows. Those eliminations follow because the four rows must place their 5s into those columns, leaving no room for 5 anywhere else in those columns.
A few practical checks
- Make sure you truly have four distinct rows and up to four distinct columns. If the rows use fewer than four columns it’s still valid (up to four), but you must cover exactly four rows.
- Confirm the confinement in the columns: if a column has a 5 in a fifth row, the pattern is broken.
- Jellyfish can be oriented either way: four rows (then eliminate in columns), or four columns (then eliminate in rows). If a pattern looks close but has an extra candidate outside the four rows/columns, it’s not a Jellyfish.
Step-by-step recognition strategy
1) Choose a digit you’re having trouble placing (common choices: 1–9 in turn).
2) Scan rows and note which columns contain that candidate in each row.
3) Look for a combination of four rows whose union of candidate columns is size four or less.
4) Check the four columns to be sure every instance of that digit in those columns appears only in the four chosen rows.
5) Eliminate that digit from any other cells in those columns that are not in the four rows.
Candidate-grid example
Below is a compact candidate-grid example showing a Jellyfish for the digit 5. In this example the four rows are 2, 4, 6 and 8; the candidate 5 in those rows appears only in columns 2, 4, 6 and 9. Because each of those four columns holds 5s only in rows 2, 4, 6 and 8, you can eliminate 5 from any other cell in columns 2, 4, 6 and 9 that lies outside rows 2, 4, 6 and 8.
Look at the candidate grid: the highlighted "Jellyfish arms" show the 5-candidates that form the fish. The other highlighted cells in the same columns (but in rows 1, 3, 5, 7 and 9) currently also contain the digit 5 as a candidate — those are legitimate eliminations once you confirm the pattern.
Quick checklist before you erase candidates
- Confirm the pattern with pencil marks (or on-screen candidates) before erasing.
- Check that you didn’t miss a 5 in one of the four columns that lies outside the chosen rows.
- After removing candidates, re-run simpler techniques: eliminations may create new singles, pairs, or chain reactions.
Final tips
Jellyfish is powerful but rarer than X‑wing or Swordfish. Use it when the same digit keeps reappearing in the same columns across multiple rows. Working systematically — digit by digit — is the most reliable approach: scan each digit’s candidate map, and you’ll spot row/column patterns. With practice, recognizing the pattern becomes faster and you’ll avoid doing unnecessary checks.
The candidate-grid below shows a clear, instructive example you can step through: confirm the four rows and four columns, then apply the listed eliminations to simplify the puzzle.