X-Wing in Sudoku: Explained with Real Examples

What an X-wing is

Pick one digit. Find two rows in which that digit has exactly two possible cells, and in both rows these cells lie in the same two columns. The four cells are the corners of a rectangle.

Each of the two rows must hold the digit once, in one of its two cells. If the top row uses the left column, the bottom row cannot use the left column too, so it uses the right one; and the other way round. Either way, the digit ends up on two opposite corners, and each of the two columns gets its digit from these two rows. No other cell of the two columns can hold the digit, so it is removed from their candidates.

An X-wing does not place a digit. It removes candidates; a digit is then placed by a single, often straight away.

The same with columns

The roles of rows and columns can be exchanged: two columns in which the digit fits only in the same two rows. Then the digit is removed from the other cells of those two rows.

Boxes play no part: the two lines of an X-wing are either two rows or two columns, wherever they lie.

How to find one

1. Write the candidates first. An X-wing is invisible without them.
2. Choose one digit and go through the rows. For each row, note the columns where the digit can still go. Only rows with exactly two places matter.
3. Look for two such rows with the same pair of columns. That is the X-wing.
4. Check whether the two columns have the digit anywhere else. If they do, remove it there; if not, the X-wing is valid but removes nothing.
5. Repeat with the columns, then with the next digit.

On the daily Sudoku, Fill notes writes every candidate at once, and selecting a cell with the digit highlights that digit in every note, which makes step 2 much quicker.

• Play the daily Sudoku
• Solve a grid step by step

Practice 1: practice puzzle 3, hard

This position comes from the third hard practice puzzle of this site, after the first 30 steps of the site’s solver. Bold digits are given; the other digits were found by simpler techniques. The small digits are the candidates still open after those steps: some were already removed by locked candidates or a naked pair, so a cell can lack a digit that its row, column and box do not rule out. At this point none of the simpler techniques on this site makes progress.

This position holds two X-wings on the 7. Either one counts as an answer, and so do both together.

The answer: in row 4 the 7 fits only in columns 5 and 8, and in row 9 also only in columns 5 and 8. Column 5 has another candidate 7 in R6C5, column 8 in R5C8 and R7C8, and this X-wing removes 7 from those three cells. The second X-wing lies in columns 3 and 4, where the 7 fits only in rows 5 and 8; it removes 7 from R5C2, R5C7 and R5C8. The site’s solver takes the first one, and row 6 then has only one place left for its 7, R6C2: a hidden single. From there, singles alone finish the puzzle.

Practice 2: an X-wing on two columns

This position comes from a hard puzzle made for this guide by the same generator and checks as the daily puzzles, after the first 26 steps of the solver. The candidates are again those still open after those steps. This time the X-wing lies in two columns.

The answer: in column 2 the 9 fits only in rows 4 and 7, and in column 9 also only in rows 4 and 7. Rows 4 and 7 therefore get their 9 from these two columns, and the 9 is removed from R4C3 and R4C7 in row 4 and from R7C1 and R7C3 in row 7. R7C1 is left with 8 alone, a naked single, and singles finish the puzzle.

Common mistakes

• A line with three places. If one of the two rows can hold the digit in a third cell, there is no X-wing: that row might use the third cell.
• Different pairs of columns. Two rows with the digit in columns 2 and 5 and in columns 2 and 7 are not an X-wing.
• Removing from the wrong lines. For an X-wing on rows, the digit is removed from the columns, never from the two rows themselves, and never from the four corners.
• Missing candidates. A candidate that was never written, or erased by mistake, can make a false X-wing appear. Fill notes on the daily Sudoku avoids that.

Where the X-wing fits

The daily puzzles are rated by the hardest technique they need. A puzzle that needs only singles is easy; one that also needs locked candidates or a naked pair is medium; one that needs a hidden pair, a naked triple or an X-wing is hard. In the daily puzzles prepared so far, from October 1, 2026 to September 30, 2029, the site’s solver needs an X-wing on 231 of the 1096 hard puzzles, about 21%.

The same idea with three rows and three columns is called a swordfish. The daily puzzles never need it, so this site does not explain it.

• All seven techniques, in order