Sudoku solving technique · Expert
Jellyfish
The X-Wing across four rows and four columns.
What is a Jellyfish?
An extension of Swordfish to four rows. When a candidate appears in only two to four cells in each of four rows, and all those cells fall within the same four columns, you can eliminate that candidate from all other cells in those four columns.
How to spot a Jellyfish
Look for four rows where a digit fits in only two to four cells, and check whether all of those cells lie in exactly four columns. Then the digit leaves those columns in every other row. The pattern is rare and hard to see on a 9×9 board; notes help. Across four columns, it works the same way.
- Difficulty
- In Sudoku Pro, this technique belongs to the Expert level.
- Learning path
- Technique 19 of 20 on the learning path
An example from the app
| Given 4 | Affected cell (orange). Candidates 1, 5, 6. Eliminated 2 | Empty | Affected cell (orange). Candidates 1, 3, 5, 6. Eliminated 2 | Affected cell (orange). Candidates 1, 5, 6. Eliminated 2 | Empty |
| Affected cell (orange). Candidates 1, 5, 6. Eliminated 2 | Given 3 | Empty | Affected cell (orange). Candidates 1, 5, 6. Eliminated 2 | Affected cell (orange). Candidates 1, 4, 5, 6. Eliminated 2 | Empty |
| Pattern (purple). Candidates 2, 3, 6 | Pattern (purple). Candidates 2, 6 | Given 4 | Pattern (purple). Candidates 2, 5, 6 | Pattern (purple). Candidates 2, 5, 6 | Given 1 |
| Pattern (purple). Candidates 1, 2, 6 | Pattern (purple). Candidates 1, 2, 6 | Given 5 | Pattern (purple). Candidates 2, 6 | Given 3 | Given 4 |
| Pattern (purple). Candidates 2, 6 | Pattern (purple). Candidates 2, 4, 6 | Given 3 | Pattern (purple). Candidates 1, 2, 6 | Pattern (purple). Candidates 1, 2, 6 | Given 5 |
| Pattern (purple). Candidates 2, 5, 6 | Pattern (purple). Candidates 2, 5, 6 | Given 1 | Given 4 | Pattern (purple). Candidates 2, 6 | Given 3 |
Small digits are candidates. Crossed-out digits can be removed in this step.
- Pattern (purple)
- Affected cell (orange)