- In the highlighted lines, 6 fits only in the highlighted cells.
- Whichever cells take 6, each crossing line gets one of them.
- So 6 can go nowhere else in the lines that cross them.
X-Wing in sudoku
An X-Wing is about one number in two rows. In each of the two rows the number can go in only two cells, and those cells line up in the same two columns, so the four cells form a rectangle. The number can then be removed from the notes of every other cell of the two columns. The same works with rows and columns swapped.
Or scan with your phone
How to spot it
Pick a number and find the rows where it is noted in exactly two cells. If two such rows have their cells in the same two columns, you have an X-Wing. Check the columns too. Highlighting one number at a time makes the rectangle stand out.
Why it works
Each of the two rows needs the number once. If the first row has it in the left column, the second row must have it in the right one, and the other way round: the two diagonals of the X. Either way each of the two columns gets the number from one of these rows, so no other cell of those columns can hold it.
On a real puzzle
This is X-Wing on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
This Extreme puzzle calls for X-Wing. Fill in the singles first. When no number is obvious any more, look for the pattern, or ask for the hint: each game has one.