Solving techniques
X-Wing
X-Wing is the first fish pattern. It looks hard, but it follows one simple counting rule.
An X-Wing happens when a digit has exactly two candidate cells in each of two rows, and those cells fall in the same two columns. The digit must go in two opposite corners of that rectangle, so it can be removed from the rest of those two columns.
The logic
Take digit 9 and two rows where 9 can go in only two cells, in the same two columns. There are two ways to place the 9s: one in each row and each column. In both, the two columns get their 9 from these two rows. So no other cell in those columns can hold a 9.
How to find an X-Wing
- Pick a digit and look at its candidates in every row.
- Find two rows where the digit has exactly two candidates.
- Check that they sit in the same two columns.
- Remove the digit from all other cells in those two columns.
The pattern works the same way with columns and rows swapped.
Common mistakes
- Using rows with three candidates. It must be exactly two in each row.
- Removing the digit from the four corner cells themselves.
- Skipping the check that the columns match exactly.
Next steps
The same idea with three rows is the Swordfish. Before this, be comfortable with pointing pairs and triples. To see the pattern on your own puzzle, use the step-by-step solver.
Frequently asked questions
What is an X-Wing in Sudoku?
A pattern in which a digit appears as a candidate in exactly two cells in each of two rows, and those cells share the same two columns.
Why is it called X-Wing?
Because the four cells form a rectangle, and its diagonals look like an X.
Can it work with columns?
Yes, with rows and columns swapped.