Look at row 2 and row 4 in this puzzle. The cells that can still contain the digit 8 in those two rows are highlighted in red.
Those 4 cells, two in each row, line up perfectly in the same two columns: column 5 and column 7.
The 4 cells form the corners of a rectangle - and if you draw its two diagonals, they cross in an X, which gives the technique its name. Rows 2 and 4 are called the base sets, and the digit 8 is called the fish digit.
There are more possible 8s in columns 5 and 7, highlighted in grey. We will come back to them.
There are only two ways for 8 to sit in rows 2 and 4, and both use opposite corners of the X-Wing.
Option 1: if R2C5 is 8, then neither R4C5 nor R2C7 can be 8, so R4C7 must be 8.
Option 2 is the mirror image: if R4C5 is 8, then R2C7 must also be 8.
Either way, column 5 gets its 8 at R2C5 or R4C5. So 8 can be removed from every other cell in column 5, in particular R5C5.
The same holds for column 7: its 8 is at R2C7 or R4C7, so 8 can be removed from R3C7.
That is what X-Wing is about: eliminating candidates with pure logic.
The possible cells for 3 in rows 6 and 9 align perfectly in the same two columns: column 4 and column 8.
They form an X-Wing, so 3 can be eliminated from all other cells in those two columns, as shown in the picture.
Here columns 5 and 7 are the base sets and 3 is the fish digit. The possible cells for 3 in these columns line up in row 2 and row 7.
They form an X-Wing, so 3 can be eliminated from all other cells in rows 2 and 7.
Scan rows and columns for each digit and find a set of two rows that align perfectly. Can you find the X-Wing on the digit 8 in this puzzle?
The base rows are R4 and R9. They hold the only possible cells for 8 in those rows, and they align in the same columns.
Lesson complete
Nice work - you went through every step. Ready for the next lesson?
Next: Swordfish Sudoku StrategyWhat X-Wing is
X-Wing is a candidate elimination pattern built on four cells that form a rectangle. When a digit has only two possible cells in each of two rows, and those cells line up in the same two columns, that digit can be removed from every other cell in those columns. The same logic works with columns and rows swapped.
The two rows (or columns) are called the base sets, and the digit is called the fish digit. X-Wing is the smallest of the fish patterns; Swordfish extends the same idea to three lines.
Row-based X-Wing example
This example follows the digit 8 in rows 2 and 4. In both rows, 8 can only go in column 5 or column 7, so the four candidate cells form a rectangle.
Another row-based example
X-Wings appear on any digit. Here the fish digit is 3 and the base sets are rows 6 and 9.
Column-based X-Wing
The base sets can also be columns. In that case the eliminations happen along the two rows where the candidates align.
Walk through this in steps 11-12 of the guided example above
How to find X-Wings
Finding X-Wings is not difficult once your pencil marks are in place. Scan the grid one digit at a time and look for two rows, or two columns, where the digit has exactly two possible cells that align.
Walk through this in steps 13-14 of the guided example above
Frequently asked questions
What does X-Wing eliminate?
An X-Wing removes the fish digit from the cells of the two crossing lines that are outside the rectangle. With row base sets you eliminate along the columns, and with column base sets you eliminate along the rows.
Does an X-Wing solve a cell directly?
Not by itself. It removes candidates, which often turns a nearby cell into a naked or hidden single.
What comes after X-Wing?
Swordfish extends the same fish logic to three rows and three columns, and XY-Wing uses bi-value cells for a different kind of elimination.
Practice next
X-Wings appear occasionally in hard and evil puzzles - rare enough to feel like a discovery, and worth scanning for when singles and pairs run out. Highlight one digit at a time and check whether its candidates align in two rows or columns.