Sudoku techniques · Extreme
Jellyfish in Sudoku
A Jellyfish is the four-line member of the fish family, after X-Wing and Swordfish. A number's candidates in four rows all fall within the same four columns, so those columns absorb it four times and lose it everywhere else.
Why it works
When a number's candidates in four rows all fall within the same four columns (or in four columns all fall within the same four rows), that candidate is removed everywhere else in those four crossing lines.
How to spot it
Four is the practical ceiling. A five-line fish is always expressible as a four-line one in the other direction.
- Each base row may hold two, three or four candidates for the number, as long as the union of their columns is four.
- Map the number's positions across all nine rows first, then look for four rows whose column sets union to exactly four.
- By the time a Jellyfish is the easiest move available, the board is genuinely Extreme.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
In each base row, 5 is a candidate in 2, 3 or 4 cells. Taking the four base rows together, every 5 candidate sits in one of only four columns. This is a Jellyfish!
- 2
The Jellyfish means we can remove the candidate 5 from the four columns that hold it, in every cell outside the four base rows.