X-Wings are when there are two lines, each having the same two positions for a number.
Take a look at this puzzle:
Once you’ve satisfied yourself that there aren’t any easy methods you can apply to move forward, take a look at the candidate positions for 6, in rows 4 and 9.
If you think about it, whichever position 6 occupies in the top row, forces the other to occupy the opposite position in the bottom row.
Here’s the clever bit – even though you don’t know which row has the 6 at the left, and which row has the 6 at the right, you know for sure that both will be occupied
And because you know that the 6 will definitely be in both of those two column positions, you can look up and down those columns, and remove any other candidates!
We can’t remove any 6s from the left hand column this time, but there are two we can remove from the right hand column, and one of those leaves an 8 as a single candidate!
What is new about this technique is that knowledge about two (similar) rows, lets you make removals from columns. Of course, it works the other way too, if you can spot similar columns.
You’ll often spot X-Wings – they are quite common, but they won’t always lead to you being able to remove candidates.
The trick to spotting X-Wings is to look for the rectangles of possible candidates. If you find four candidates on the corners of a rectangle, check to see if they are an X-Wing for both rows and columns – that might save you some extra time!
X-Wing in rows for 8.
X-Wing in rows for 9.
X-Wing in columns for 4.