How to Solve Sudoku: Jellyfish

The Swordfish with 4 lines. If, in 4 rows, the places for a digit fit in the same 4 columns, it can be removed from the other cells of those 4 columns.

On Puzzle Lounge it appears at Master.

See it step by step

  1. A board full of notes. Look only at the 9s (the small digits are the notes, or candidates)
  2. ● = a cell where 9 can go (notes other than 9 have been removed)
  3. In row 1, 9 is only in columns 5 and 9
  4. Rows 3, 6 and 7 also fall only in columns 1, 5, 8 and 9 (the ● of 4 rows fit in 4 columns)
  5. 9 can only go in these 8 ways (each puts one in columns 1, 5, 8 and 9)
  6. In every case, the 9s of columns 1, 5, 8 and 9 are in these 4 rows
  7. So the other 9s can be removed (9 goes from row 9, column 5, row 5, column 9 and row 2, column 1)

This is Master No.185, partly solved.

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How to find it

  1. Pick a digit and list the rows where it has only 2 to 4 candidates.
  2. Among those, find 4 rows whose candidates fit in 4 columns in all.

Things to watch

  • There are many sets of 4 rows, so this is hard to find. It is the last resort when the X-Wing and Swordfish turn up nothing.
  • When it holds in 4 rows, the same pattern can sometimes be found in the remaining 5 columns.
Read the full explanation

This is the 4-line version of the X-Wing (2 lines) and Swordfish (3 lines). If a digit's candidates, in 4 rows, all lie within the same 4 columns, the digit can be removed from the other rows of those 4 columns.

Suppose a digit's candidates, in each of rows A, B, C and D, lie only in columns W, X, Y and Z (2 to 4 places in each row).

The digit goes once in each of the 4 rows, so it goes once in each of the 4 columns.

So the digit cannot go in the other rows of columns W, X, Y and Z.

In rows 1, 3, 6 and 7, the candidates for 9 lie only in columns 1, 5, 8 and 9. So the 9s of columns 1, 5, 8 and 9 are used up in these 4 rows, and 9 can be removed from the candidates at row 9, column 5, row 5, column 9 and row 2, column 1.

Other techniques

Hidden SingleNaked SinglePointingClaimingHidden PairHidden TripleCage Combinations (Killer)Rule of 45 (Killer)Naked PairNaked TripleX-WingXY-WingSwordfishSkyscraper2-String KiteXYZ-WingW-Wing

For the basics and how to use notes, see how to solve sudoku, with tips. If you get stuck, the game's hints guide you step by step, starting with where to look.

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