sudokUI › Sudoku techniques › X-Wing
X-Wing in sudoku
2-fish · Naked Pair seen sideways · 4-candidate X-Cycle · how it fits
Digit X has exactly two candidates in each of two rows, all four lying in the same two columns, or likewise with rows and columns swapped.
Why it works
Each row places X in one of the two columns, and the two rows cannot share a column, so each column gets its X from these four cells. Remove X from all other cells in those columns, or in those rows for the swapped form.
How to spot it
Filter on one digit and look for two rows, or two columns, whose only two candidates form the corners of a rectangle.
Worked example
X-Wing on 7: in rows 3, 8, 7 can only go in columns 3, 8 (r3c3, r3c8, r8c3, r8c8). Those 2 rows will use up 7 in those 2 columns, so 7 can go nowhere else in them. So 7 is removed from r2c3.
Practise it on a real puzzle. sudokUI generates a puzzle that needs X-Wing and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs X-Wing, the puzzle's difficulty rating grows by 140. Its class is Hard. It is needed in 1 in 19 puzzles that sudokUI generates. How the rating works.