X-Wing: Sudoku Technique Explained

X-Wing: sudoku technique

A rectangle of four candidates that lets you clear a number from two whole lines.

How it works

Find a number that has exactly two possible places in one row, and exactly two in another row, in the same two columns. These four cells form the corners of a rectangle.

The number must go in opposite corners, one way or the other. Either way, both columns get their copy of the number from these corners. So every other cell in those two columns can lose that candidate. Swap rows and columns and the logic still holds.

Worked example

This example comes from a real puzzle in our collection, at the exact moment the technique applies.

Here is the board at that moment, with the candidates still open in each empty cell. Earlier steps may already have ruled some out.
4
2
5
6
8
3
7
19
19
7
6
3
4
1
9
58
2
58
1
8
9
5
2
7
3
6
4
8
1
2
9
35
6
45
345
7
59
4
7
2
35
8
6
1359
1359
3
59
6
7
4
1
59
8
2
59
3
14
18
6
45
2
7
89
6
7
8
13
9
2
145
345
35
2
59
14
138
7
45
189
39
6
In two lines, the number has its only spots in the same two crossing lines. The cells that matter are highlighted.
4
2
5
6
8
3
7
19
19
7
6
3
4
1
9
58
2
58
1
8
9
5
2
7
3
6
4
8
1
2
9
35
6
45
345
7
59
4
7
2
35
8
6
1359
1359
3
59
6
7
4
1
59
8
2
59
3
14
18
6
45
2
7
89
6
7
8
13
9
2
145
345
35
2
59
14
138
7
45
189
39
6
The result: one candidate goes. 9 leaves row 9, column 8.
4
2
5
6
8
3
7
19
19
7
6
3
4
1
9
58
2
58
1
8
9
5
2
7
3
6
4
8
1
2
9
35
6
45
345
7
59
4
7
2
35
8
6
1359
1359
3
59
6
7
4
1
59
8
2
59
3
14
18
6
45
2
7
89
6
7
8
13
9
2
145
345
35
2
59
14
138
7
45
189
39
6

Where you'll need it

Every expert board here needs an X-Wing or its larger cousin at some point. Scan one number at a time with your pencil marks complete.

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