Jellyfish

A Jellyfish is the four-line member of the fish family that starts with the X-Wing and the Swordfish. Pick one digit and four rows in which that digit can only go in the same four columns, each row using two, three or four of them. The four rows need four copies of the digit, one in each row, and those copies can only sit in the four columns, one per column. So the four columns get their copy from these rows, and the digit can be removed from every other cell of the four columns. The same works with rows and columns swapped. The Jellyfish is an Expert technique in Bare Sudoku.

When to use it

When X-Wings and Swordfish find nothing for a digit that still has many candidates spread over the board. Jellyfish are rare, because smaller fish, pairs and other patterns usually solve the digit first, but they do appear in hard puzzles. A full set of notes is needed.

How to spot it

Choose a digit and list, for each row, the columns where it can still go. Look for four rows whose lists together use only four columns; a row may use two, three or four of them, but none may use a column outside the four. Every candidate of the digit in those columns outside the four rows can then be removed. Check columns against rows the same way.

Example

Jellyfish
c1c2c3c4c5c6c7c8c9
r113593541598215976
r262871459591454593
r3159715314568249
r4435935682597691
r571926194385
r681595615937246949
r726945734518
r8358745961493452
r91354135259863597

Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.

  1. In rows 2, 4, 5, 9, digit 9 can only go in columns 2, 5, 6, 8: its candidates there are r2c5, r2c6, r2c8, r4c2, r4c6, r4c8, r5c2, r5c5, r9c5, r9c8.
  2. Each of the four rows needs a 9, and it must be in one of the four columns. Four rows, four columns: the 9s of these rows take exactly one cell in each of the four columns.
  3. So every column of 2, 5, 6, 8 gets its 9 from one of rows 2, 4, 5, 9.
  4. No other cell of those columns can be 9. Remove 9 from r6c2, r6c8.

Common mistakes

In Bare Sudoku

Needed from level: Expert
In the game, the hint says: Jellyfish

Play this puzzle · Play Sudoku · Sudoku solver

Practice puzzles

In these puzzles, Jellyfish is the hardest technique you need. Each link opens the puzzle in the game; the solver link shows every step.

  1. Expert 5.2 · Sudoku solver
  2. Expert 5.2 · Sudoku solver
  3. Expert 5.2 · Sudoku solver
  4. Expert 5.2 · Sudoku solver
  5. Expert 5.2 · Sudoku solver
  6. Expert 5.2 · Sudoku solver
  7. Expert 5.2 · Sudoku solver
  8. Expert 5.2 · Sudoku solver
  9. Expert 5.2 · Sudoku solver
  10. Expert 5.2 · Sudoku solver

Frequently asked questions

What is a Jellyfish in Sudoku?

A fish pattern of size four: one digit confined to the same four columns in four rows, or to the same four rows in four columns. The digit is removed from the rest of those columns or rows.

How is a Jellyfish related to the X-Wing and the Swordfish?

All three use the same idea with a different number of lines: two for the [X-Wing](x-wing), three for the [Swordfish](swordfish) and four for the Jellyfish. Larger fish are possible, but each one comes with a smaller fish in the other direction, so four lines is the largest size worth looking for.

Does every row need the digit in all four columns?

No. Each row may hold the digit in two, three or four of the columns, as long as no row holds it outside them.

Is the Jellyfish common?

No. Most puzzles that could use one are solved first by smaller fish or other patterns. When the hint names a Jellyfish, the puzzle is usually a hard Expert one.

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