Sudoku technique
Jellyfish
The X-Wing stretched to four rows and four columns. Same logic: the digit gets eliminated on the four columns involved, outside the four-row pattern.
A puzzle that requires it
Level Extreme — the puzzle is shown twice: right below, and again in the demo that follows.
| 6 | 2 | 7 | ||||||
| 5 | 4 | 1 | ||||||
| 9 | ||||||||
| 9 | 8 | 6 | ||||||
| 2 | 4 | 9 | 3 | |||||
| 3 | 4 | 7 | ||||||
| 4 | ||||||||
| 3 | 1 | 6 | ||||||
| 3 | 7 | 2 |
Demonstration
State of the grid right when the technique applies, after 18 earlier deductions.
Demo — watch first, you'll play right after.
- Technique's pattern
- Candidates removed
- Solved earlier
- Given (clue)
| 6 | 2 | 7 | 3 | |||||
| 2 | 8 | 9 | 5 | 4 | 1 | |||
| 9 | 8 | 8 | ||||||
| 7 | 9 | 8 | 4 | 6 | ||||
| 2 | 4 | 7 | 6 | 9 | 3 | |||
| 6 | 3 | 9 | 4 | 7 | ||||
| 6 | 8 | 9 | 1 | 3 | 4 | |||
| 3 | 1 | 8 | 6 | 4 | ||||
| 4 | 9 | 3 | 7 | 1 | 2 | 6 |
Jellyfish
Le 8 n'occupe que 4 colonnes sur ces 4 lignes, et elles s'alignent. Quelle que soit la répartition, ces 4 colonnes ont déjà leur 8 à l'intérieur du motif : il disparaît partout ailleurs sur elles.
Candidates removed:
- the 8 from cell L3C2
- the 8 from cell L2C3
- the 8 from cell L3C3
- the 8 from cell L7C3
- the 8 from cell L8C3
The Jellyfish is the last technique in the catalogue, and the largest of the fish family. When a digit is possible in only two, three or four cells on each of four rows, and all those cells fall in the same four columns, that digit is eliminated from the rest of those four columns.
The principle
Choose a digit, say 6. Find four rows where 6 is possible in only two, three or four cells, and where all those cells are in the same four columns, say columns 2, 4, 7 and 8. It is the reasoning of the X-Wing and the Swordfish, stretched to four rows and four columns. The rows do not need to have the 6 in all four columns.
Why does it work? Each of the four rows must contain a 6, and that 6 can only be in one of the four columns. That makes four 6s to place. A column holds only one 6: those four 6s therefore fill four different columns, that is columns 2, 4, 7 and 8, once each.
You do not know which row takes which column, but you know those four columns already have their 6 inside the pattern. You can therefore remove the 6 from every other cell of those four columns.
How to spot it
You need candidate notes. Take one digit at a time and find the rows where it appears in two to four cells. Look for four of those rows whose cells together occupy exactly four columns. If they occupy five, it is not a Jellyfish.
The pattern is rare, and slow to check by hand: you have to hold four rows and four columns at once. Then run the same search with rows and columns swapped.
A worked example
For the 6, row 1 allows it only in columns 2 and 4, row 3 only in columns 4 and 7, row 6 only in columns 7 and 8, row 9 only in columns 2, 7 and 8. Together, those four rows use only columns 2, 4, 7 and 8: it is a Jellyfish, although no row has the 6 in all four columns.
Each row puts its 6 in one of those columns, and each column receives only one. In column 4, the cell in row 5 has candidates 3 and 6. Remove the 6, 3 remains, and you can place it. Do the same in columns 2, 7 and 8 for the cells of rows 2, 4, 5, 7 and 8 that still hold the 6. The cells of rows 1, 3, 6 and 9 do not change.
Common mistakes
The most common mistake is to eliminate in the wrong place. When the pattern is formed by four rows, you clean the columns, never the other cells of those four rows: that is where the 6 will end up.
Another trap is to accept four rows whose cells occupy five columns. The four rows must fit together in four columns, not one more.
When to use it
The Jellyfish is expected at the Extreme level, along with the XY-Wing, the XYZ-Wing and the Swordfish. It is a rare pattern, kept for the highest levels of the site: look for it last, when no simpler technique works. There is no next technique in the catalogue. To review the method, visit How to solve a sudoku, then test yourself at the Extreme level.
- Extreme sudoku
- How to solve a sudoku
- Technique: the Swordfish
- Technique: the X-Wing
- Learn the techniques
Frequently asked questions
Why does the Jellyfish eliminate the digit in the columns?
Because each of the four rows of the pattern must have that digit, and their possible places are in the same four columns. Each of those columns gets its digit inside the pattern: it can no longer appear elsewhere in those columns.
Must every row have the digit in all four columns?
No. A row may have the digit in only two or three of the four columns. What matters is that all the cells of the four rows fit in the same four columns.
Can a Jellyfish also be built with columns?
Yes. If the digit is possible in only two to four cells on each of four columns, and those cells fall in the same four rows, remove it from the other cells of those four rows.
Does a Jellyfish place a digit?
No, it removes candidates. Those eliminations can then leave a Naked or Hidden Single, which lets you place a digit.
Practice
Every technique helps remove candidates ; once a cell has only one left, you place it.
Three puzzles where Jellyfish is the key move, served right before the step, with a coach.
Puzzle 1
Extreme
The technique is used once in this puzzle
Puzzles change every day.