X-Wing: the Sudoku technique explained

How it works

Take two rows where a digit, say 4, has exactly two possible cells each, in the same two columns. That makes four cells at the corners of a rectangle. Each row needs one 4. If the top row has its 4 in the left corner, the bottom row must have its 4 in the right corner, and the other way round. The two 4s always land on a diagonal, which is the X in the name.

Either way, each of the two columns gets its 4 from one of the corners. So no other cell in those two columns can be a 4.

The same works turned on its side: two columns with the digit in the same two rows clear the rest of those rows.

1. Choose a digit and look for rows where it has exactly two candidate cells.
2. Find two such rows whose candidates sit in the same two columns.
3. Remove the digit from every other cell of those two columns.
4. If you find nothing, repeat with columns in place of rows.

A real example

The four blue cells are the corners. The digit is removed from the red cell, which lies in one of the two columns of the rectangle but outside its two rows.

How to spot them

X-Wings are found by digit, not by cell. Highlight one digit at a time, which Sudoku Clan does when you select a cell holding it, and look at the rows where it has only two spots left. Two rows with matching columns is all you need. Digits that are already placed four or five times are the best candidates, because their remaining positions are few.

Common mistakes

• Using a row where the digit has three candidates. Both base rows need exactly two for an X-Wing.
• Removing from the wrong direction. If you found the pattern in rows, you remove along the columns.
• Removing a corner. The four corner cells keep the digit.

An X-Wing on three rows and three columns is a Swordfish.