Solving techniques
Swordfish
Swordfish is X-Wing with one more line. It is rare, and when it appears nothing else works.
A Swordfish happens when a digit has two or three candidate cells in each of three rows, and all those cells lie in the same three columns. The digit must go in those columns inside those rows, so it can be removed from the rest of those three columns.
The logic
The reasoning from X-Wing extends to three lines. Three rows each need the digit once, and all candidate cells are confined to three columns. Those three columns will get their digit from those three rows, so no other row can use them.
How to find a Swordfish
- Pick a digit and list rows where it has two or three candidates.
- Look for three such rows whose candidates fall within the same three columns.
- Remove the digit from all other cells in those three columns.
Not every row needs three candidates. Some may have two. What matters is that all candidates together touch only three columns.
Common mistakes
- Requiring three candidates in each row.
- Counting columns incorrectly. They must be three in total, not three per row.
- Removing candidates from the pattern cells themselves.
Related
Learn X-Wing first. The next level uses chains: XY-Wing and simple coloring. All of these appear in Expert puzzles.
Frequently asked questions
What is a Swordfish in Sudoku?
A pattern in which a digit is confined to the same three columns across three rows.
Is it harder than X-Wing?
Yes. It involves more cells and is harder to see by eye.
How often does it appear?
Rarely. Only some Expert puzzles need it.