How to solve an X-Wing in Sudoku
An X-Wing is a digit that has exactly two spots in each of two rows, in the same two columns, which clears it from the rest of those columns.
How it works
Suppose a digit can go in only two cells of one row, and in only the same two columns of another row. The four cells form a rectangle.
The digit must take opposite corners of the rectangle, so it will fill both columns. It can be removed from every other cell in those two columns. The same works with rows and columns swapped.
How to spot it
Pick one digit and look for two rows where it has exactly two candidates, in matching columns.
A worked example
This is a step from a real puzzle, a moment where the next move needs an X-Wing.
Struck-through digits are the ones that can be removed.
7 is limited to the same 2 columns across 2 rows (R3C6, R3C9, R8C6, R8C9). So 7 can't appear elsewhere in those columns: remove it from R1C6, R9C6, R1C9, R9C9.
Where it fits in Rocksled Sudoku
Puzzles that need an X-Wing are never rated below Very Hard. Each step of this kind adds 32 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.