When a digit's places in four rows all fall inside the same four columns, the four rows fill those columns with that digit. Every other cell in those columns loses it. A row can have two, three or four places, and the same holds with rows and columns swapped.
How to spot it
A jellyfish reads one digit at a time, like the X-wing and the swordfish. It takes four rows and four columns.
Pick a digit and note, for every row, the columns where it can still go. Keep the rows that give two, three or four places. Then look for four of those rows whose places, taken together, use only four columns. A row does not need a place in all four. It only must not reach a fifth.
Start with the digits that are placed once or not at all. A jellyfish only adds something when its digit is still open in eight or nine rows. With fewer open rows, the same strike also shows up as a smaller fish, and you would find that one first.
Count the 6s row by row
Take the 6. It is placed once, in row three, so eight rows still need one. Row five leaves it seven places and row seven five. Rows two and six leave it four each. Row one has six empty cells, and the 6 fits in three of them: columns four, six and seven.
Add rows that stay inside four columns
Row four uses the same three columns as row one. Row nine has three places, in columns two, six and seven, which brings in column two. That makes four columns: two, four, six and seven. Row eight fits too, since its only places are columns two and seven. Rows two and six have four places each, but row two reaches columns five and eight and row six reaches columns one and five. Both stay out.
See why each column gets a 6
Each of the four rows needs a 6, and each can only put it in column two, four, six or seven. That is four 6s going into four columns, and a column holds one 6. So each of the four columns receives exactly one, from one of these rows. You do not need to know which row fills which column.
Clear the 6 from the rest of the four columns
Columns two, four, six and seven each take their 6 from the pattern, so no other cell in those columns can hold one. Eight marks go. In column two, rows five and six. In column four, rows two, five, six and seven. In column six, row five, and in column seven, row two. The eleven cells of the pattern keep their 6s.
Read what the strike left behind
In column four, the cell in row seven showed only 4 and 6. Without the 6 it is a 4, a naked single. Before the strike this grid had no single of either kind. Nothing simpler removed a mark either, the X-wing and the swordfish included. On the path, the jellyfish is the first technique that does anything here.
Why the rows can be uneven
The argument counts rows against columns. Four rows each owe the grid one 6, and all of their places lie inside four columns. Four 6s in four columns, one per column, means those columns are full. The spread does not matter. Here row eight has two places and the other three rows have three each. A row could have all four.
What breaks it is a fifth column. If any of the four rows can put its 6 outside the set, that row might fill none of the four columns, and the count no longer holds.
On this grid the 6 goes in the seventh cell of row one, the fourth cell of row four, the second cell of row eight and the sixth cell of row nine. That is one per column, as the argument said. You can make the whole strike without knowing it.
Rows and columns can swap. If a digit's places in four columns all fall inside the same four rows, it leaves every other cell of those rows. The question at the end of this page uses a column jellyfish.
Four rows, five columns
Rows one, four, seven and nine each have two or three places for a 7. In row one they are columns one, two and three. In row four, three and six. In row seven, one, three and five. In row nine, two, three and six. Column three turns up in all four rows, and at a glance it looks like a jellyfish.
Now count the columns. One, two, three, five and six make five. Four rows can place their 7s across five columns and leave one of them out, so no column is guaranteed. Drop column six and treat the rest as a jellyfish on columns one, two, three and five, and you would strike the 7 from four cells of row six. One of them is the second cell, and on this grid that is where row six's 7 goes.
Before you strike, list the columns from every row. If the list runs to five, there is nothing to remove.
Columns four, six and seven, the same three again, then two and seven, then two, six and seven. Four columns in all. Each of them gets its 6 here.
Columns one, two and three, then three and six, then one, three and five, then two, three and six. Five columns. Nothing can be struck.
The same cells from the columns
After the strike, column two has the 6 in rows eight and nine, and column four in rows one and four. Column six has it in rows one, four and nine, and column seven in all four rows of the pattern. Read from the columns, the same eleven cells are a jellyfish again. It removes nothing this time, because rows one, four, eight and nine have no other cells with a 6.
The jellyfish is the largest fish on the path. A fish on five rows always has a partner of four lines or fewer in the other direction, built the way the question below builds one, so four is as large as a useful fish gets.
Work one digit at a time, starting with digits placed once or not at all. Keep the rows where it has two to four places, and look for four whose places share the same four columns. If they do, the digit leaves the rest of those columns. A fifth column anywhere and there is nothing to strike.
Where this sits on the path
A jellyfish places nothing. It removes candidates and leaves the placing to the singles. On the grid above it made one naked single, in column four.
It is the third and last of the fish. The X-wing locks a digit with two rows, the swordfish with three and the jellyfish with four. That is also why jellyfish are rare. Say the digit is already placed twice, so seven rows are open. The four rows of the pattern fill four columns, which leaves three open columns, and those can only take the digit from the three open rows outside the pattern. That is a swordfish or something smaller, read from the columns, and it strikes the same cells. A jellyfish only adds something when its digit is open in eight or nine rows, as the 6 is here.
The Y-wing comes next and starts the wings.
Columns one, five, eight and nine each have the 6 in two to four places. Column one uses rows five, six and seven. Column five uses rows two, five, six and seven, column eight rows two, five and seven, and column nine rows five and seven. Which 6s can you strike?
List the rows the four columns use. Those are the lines the columns fill with a 6, and that is where the strike happens.
Recap
- Look for
- One digit with two to four places in each of four rows, all inside the same four columns.
- Scope
- Four rows, with the strike in their four columns. Or four columns, with the strike in their four rows.
- Result
- The digit comes out of every other cell in those four columns. The pattern's cells keep it.
- Tell-tale
- A digit placed once or not at all, whose places keep landing in the same few columns.
- If it fails
- If the four rows reach a fifth column, there is no jellyfish. Look for a swordfish or an X-wing among fewer of them, try columns as the base, or move to the next digit.