X-Wing
Advanced Removes candidates Used in: Sudoku, Killer Sudoku
A digit’s candidates in two rows line up in the same two columns, forming a rectangle. The digit must occupy those two columns within those rows, so it can be removed from the rest of those columns.
How to spot it
A digit confined to the same two columns in two different rows (or two rows in two columns).
What you can conclude
Remove the digit from those two columns everywhere outside the rectangle.
Worked example
6 sits only in columns 2 and 7 within rows 1 and 6. Wherever it lands, those columns are covered — so erase 6 from columns 2 and 7 in every other row.