Step 22 · Master
What is Jellyfish? How to Solve 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
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
Think about 9 only. In column 3, column 4, column 7, and column 8, every spot for 9 (R2C3, R9C3, R2C4, R5C4, R9C4, R2C7, R8C7, R9C7, R5C8, and R8C8) is inside row 2, row 5, row 8, and row 9. Each of those four lines needs one 9, so the 9s of row 2, row 5, row 8, and row 9 are used up here. So the other cells of row 2, row 5, row 8, and row 9 (R2C6, R2C9, R5C1, R5C2, R5C9, R8C6, and R9C5) can't be 9.