Step 26 · Master
What is Jellyfish? How to Solve Killer Sudoku
The four-line version of X-Wing: when a digit's candidates in four rows all fall within the same four columns, you can remove that digit from the other cells of those columns.
What it is
The four-line version of X-Wing (two lines) and Swordfish (three). If a digit's spots in four rows all fall within the same four columns, remove it from the rest of those columns.
How to spot it
Pick one digit, collect four rows where it has two to four spots, and check whether they use only four columns in total.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
Think about 2 only. In row 1, row 3, row 5, and row 7, every spot for 2 (R1C8, R1C9, R3C2, R3C7, R3C8, R5C7, R5C8, R5C9, R7C2, R7C8, and R7C9) is inside column 2, column 7, column 8, and column 9. Each of those four lines needs one 2, so the 2s of column 2, column 7, column 8, and column 9 are used up here. So the other cells of column 2, column 7, column 8, and column 9 (R8C2, R8C8, R9C8, R8C9, and R9C9) can't be 2.