Step 10 · Expert
What is Swordfish? How to Solve 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
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 2 only. In row 3, row 6, and row 9, every spot for 2 (R3C4, R3C5, R3C9, R6C4, R6C9, R9C4, and R9C5) is inside column 4, column 5, and column 9. Each of those three lines needs one 2, so the 2s of column 4, column 5, and column 9 are used up here. So the other cells of column 4, column 5, and column 9 (R1C4, R1C5, R1C9, R2C9, and R4C9) can't be 2.