The X-Wing

The X-Wing is the first "fish" pattern most players learn. It looks at two rows and two columns at once, and it is often the key move in Expert and Master puzzles.

The pattern

Pick a number. Find two rows where that number has exactly two candidates each, and where those candidates sit in the same two columns. The four cells form the corners of a rectangle.

In the first row, the number must go in one of its two corners. The same is true for the second row. Either the number fills the top-left and bottom-right corners, or the top-right and bottom-left corners. In both cases, each of the two columns gets exactly one copy of the number from these corners.

That means no other cell in those two columns can contain the number. You can remove it from the rest of both columns.

The pair / patternCandidates removedUnit being checked

In the example, the second row and the eighth row each have only two places for a 4, and in both rows those places are in the third and seventh columns. Whichever way the corners resolve, columns three and seven each receive their 4 from these rows. So the 4 candidates elsewhere in those columns, shown in red, can be removed.

Rows or columns

The X-Wing works the other way round as well. If two columns each have a number in exactly the same two rows, you can remove that number from the rest of those two rows.

How to find an X-Wing

1. Choose one number and look only at its candidates.
2. List the rows where that number appears exactly twice.
3. Check whether two of those rows use the same pair of columns.
4. If they do, look for that number elsewhere in those columns. If there is none, the X-Wing is valid but doesn't help yet.

If you can't find an X-Wing, double-check the simpler techniques first. A missed pointing pair or naked pair is far more common than a missing X-Wing.

Play Sudoku now →