Swordfish
Swordfish is X-Wing extended to three lines: a digit that in three rows can only go in the same three columns can be removed from the rest of the columns.
Learn the techniqueJellyfish
Jellyfish is the fish above Swordfish: four rows where a digit can only go in the same four columns.
See the technique in practice
Work through the examples step by step. Each step explains what you see on the puzzle and why the conclusion holds.
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.
- Thus the spot for 2 in the four columns is used up by the pattern. We remove 2 from the notes in row 8, column 4 and row 8, column 7.
How to recognize the pattern
Jellyfish is the fish above Swordfish and spans four base rows and four cover columns. You look for one digit where all possibilities in four rows lie within the same four columns. Like in the smaller fish, each row does not need to use all columns, two or three possibilities per row is 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.
Step-by-step procedure
- 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.
Common mistakes
- 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.
When do you need the technique?
Puzzles at Extreme level require techniques that combine three or more cells in logical reasoning of the if-then type. Try your way through the examples below, step by step, using the same tools the solver uses on your own puzzle.
Try it on your own puzzle
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