345
Techniques · Hard
X-Wing
Take a digit that has exactly two places in each of two rows, and the places line up in the same two columns. The four cells form a rectangle. Whichever way the rows resolve, the digit takes one cell in each of those columns, so it can be removed from every other cell in the two columns. The same works with columns and rows swapped.
How to spot it
Pick a digit and look for rows where it has only two places. If two such rows share the same two columns, you have an X-Wing. Then check the other cells of those columns for the digit.
Example
9
34
458
14
18
6
2
7
124
124
8
9
7
6
3
5
14
145
7
6
45
2
3
148
19
89
1279
12
129
6
3
1579
178
4
589
6
8
5
47
14
179
2
179
3
1479
3
149
2
8
1579
17
6
59
2489
24
249
1
5
78
479
3
6
1348
6
134
78
9
2
5
17
14
19
5
7
3
6
4
19
8
2
In columns 4 and 8, 7 can only go in rows 5 and 8, so 7 can be removed from the rest of rows 5 and 8 (X-Wing).
the pattern cells that lose a candidate (crossed out)