Sudoku Technique
X-Wing is one of the first advanced sudoku techniques many evil sudoku players learn. It does not place a number directly; it removes candidates so the puzzle can open up again.
Open a hard board, keep clean notes, and look for candidate rectangles after the easy logic dries up.
An X-Wing starts with one candidate number. If that candidate can appear in exactly two cells in one row, and exactly two matching columns in another row, the four cells form a rectangle. The same logic also works with two columns and matching rows.
Because the candidate must occupy one corner in each of the two lines, it cannot appear anywhere else in the paired columns or rows. Those outside candidates can be removed.
Evil sudoku often reaches a crowded middle game where no single number is obvious. X-Wing is valuable because it creates progress without guessing. One good elimination can trigger a new single, a locked candidate, or a pair that was hidden before.