X-wing
ExpertTake one digit. If it has exactly two possible places in one row, and exactly two in another row, in the same two columns, the four cells form a rectangle. The digit goes in two opposite corners of it, so the rest of those two columns can’t hold it. The same works with columns and rows swapped.
How to spot it
- Pick one digit and look only at its notes.
- Find two rows where it has exactly two places, in the same two columns.
- Remove the digit from every other cell of those two columns.
Why it works
Each of the two rows needs the digit once, and has only those two places for it. Whichever corner the first row uses, the second row uses the other column, so both columns get their digit from these rows.
Example
9
2
4
6
2
8
3
5
6
7
9
5
9
1
8
1
7
5
1
5
6
1
9
8
8
5
7
4
3
4
8
6
Look at the 1s in row 3 and row 8.
Common mistakes
- Using a row where the digit has three places: then it is not an X-wing (it may be part of a swordfish).
- Removing the digit from the two rows instead of the two columns.