Jellyfish: learn the sudoku technique step by step at Sudokuen

See the technique in practice

Work through the examples step by step. Each step explains what you see in the puzzle and why the conclusion is valid.

Example:

We follow digit 2 through four rows, namely row 1, row 2, row 7 and row 9. The notes show where 2 can still go.
Here is the pattern: all the highlighted cells lie in the same four columns, namely column 4, column 5, column 7 and column 8.
Each of the four rows must get digit 2 in one of its highlighted cells. Two cannot use the same column, because then it would get 2 twice. So each takes its own.
The pattern therefore accounts for all positions of 2 in the four columns. We remove 2 from the notes in row 8, column 4 and row 8, column 7.

We follow digit 4 through four rows, namely row 1, row 4, row 7 and row 8. The notes show where 4 can still go.
Here is the pattern: all the highlighted cells lie in the same four columns, namely column 1, column 2, column 3 and column 7.
Each of the four rows must get digit 4 in one of its highlighted cells. Two cannot use the same column, because then it would get 4 twice. So each takes its own.
The pattern therefore accounts for all positions of 4 in the four columns. We remove 4 from the notes in row 3, column 2 and row 3, column 3.

We follow digit 6 through four rows, namely row 3, row 5, row 6 and row 8. The notes show where 6 can still go.
Here is the pattern: all the highlighted cells lie in the same four columns, namely column 3, column 4, column 5 and column 6.
Each of the four rows must get digit 6 in one of its highlighted cells. Two cannot use the same column, because then it would get 6 twice. So each takes its own.
The pattern therefore accounts for all positions of 6 in the four columns. We remove 6 from the notes in row 4, column 5, row 4, column 6 and row 9, column 3.

We follow digit 9 through four columns, namely column 2, column 3, column 7 and column 9. The notes show where 9 can still go.
Here is the pattern: all the highlighted cells lie in the same four rows, namely row 1, row 3, row 6 and row 7.
Each of the four columns must get digit 9 in one of its highlighted cells. Two cannot use the same row, because then it would get 9 twice. So each takes its own.
The pattern therefore accounts for all positions of 9 in the four rows. We remove 9 from the notes in row 6, column 5.

We follow digit 8 through four rows, namely row 2, row 6, row 7 and row 9. The notes show where 8 can still go.
Here is the pattern: all the highlighted cells lie in the same four columns, namely column 4, column 5, column 8 and column 9.
Each of the four rows must get digit 8 in one of its highlighted cells. Two cannot use the same column, because then it would get 8 twice. So each takes its own.
The pattern therefore accounts for all positions of 8 in the four columns. We remove 8 from the notes in row 5, column 9.

Jellyfish is the next fish pattern after Swordfish and spans four base rows and four cover columns. Look for a digit whose possible positions in four rows all fall within the same four columns. As with smaller fish patterns, each row does not need to use every column; two or three positions per row are enough.

The four placements must distribute across each row and each column, so each of the four columns gets its copy of the digit from the base rows. Therefore, the digit can be removed from all other cells in the four columns. Jellyfish is rare, so if you suspect one, it is worth double-checking if an easier pattern explains the same thing.

Choose a digit and write down for each row which columns the digit can go in.
Find four rows where all their possibilities together lie in only four columns.
Check that none of the four rows has possibilities outside the four columns.
Remove the digit from all other cells in the four columns.

Losing track of the columns. With four rows and four columns, it is easy to lose count, so write down the column numbers as you check.
Missing an easier fish. An X-Wing or Swordfish in the same area often gives the same removals with much less work.
Accepting a row with one possibility outside the four columns. One candidate out of place is enough to invalidate the whole pattern.

Puzzles at the Extreme level require techniques that combine three or more cells in if-then chains of logic. Work through the examples below step by step, using the same tools that the solver applies to your own puzzle.

Try the technique in the Sudoku Solver

Jellyfish

See the technique in practice

How to recognize the pattern

Step-by-step procedure

Common mistakes

When do you need the technique?

Related techniques