Sudoku
X-Wing practice
An X-Wing is built on one number and four cells at the corners of a rectangle. In two different rows, the number can only go in the same two columns.
The number must land on two opposite corners. Either way each of the two columns gets its copy from one of those rows, so the number can be crossed off every other cell in both columns. The pattern also works turned on its side, with two columns and two rows.
How to spot a X-Wing
Pick a number and look for rows where it has exactly two places left. When two such rows use the same two columns, you have an X-Wing. Filling every pencil note first makes this far easier to see.
Practise it on this page
The board above deals positions where the X-Wing is the only way forward, with every pencil note filled in. Cross off the candidates it rules out and press Next. Press Hint to see the pattern marked on the board.
In a full game, Expert is the first level that can ask for it.