Step 14 · Expert
What is Swordfish? How to Solve Killer Sudoku
The three-line version of X-Wing: when a digit's candidates in three rows all fall within the same three columns, you can remove that digit from the other cells of those columns.
What it is
An X-Wing stretched to three lines. Think about one digit (say 6).
① In three rows, 6 fits in only two or three cells each.
② All of those cells fall within the same three columns.
Each of the three rows needs one 6, so the 6s of those three columns are used up here. Remove 6 from the rest of those columns.
How to spot it
Pick one digit, collect three rows where it has two or three spots, and check whether they use only three 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 1 only. In column 2, column 6, and column 8, every spot for 1 (R7C2, R8C2, R8C6, R9C6, R7C8, R8C8, and R9C8) is inside row 7, row 8, and row 9. Each of those three lines needs one 1, so the 1s of row 7, row 8, and row 9 are used up here. So the other cells of row 7, row 8, and row 9 (R7C9, R8C9, and R9C9) can't be 1.