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Gradually it's getting interesting. X-Wing is considered a moderately difficult higher method. As with Blocked Blocks, the X-Wing method depends on the perspective: seen from either the row or the column.

Seen from the line: If a candidate in two lines can only be in the same two columns, then it can be deleted from all fields in the columns except the ones in the two lines.
Seen from the column: If a candidate in two columns can only be in the same two lines, then it can be deleted from all fields in the lines except the ones in the two columns.

The reasoning can be better explained using an example. Consider the following picture and pay attention to the green and pink fields only.

Here are two lines (3 + 7) which can contain the candidate 5 in only two places. Coincidentally in the same two columns (5 + 9).

This construction can only be solved crosswise. In the end either the pink fields or the green fields are 5 - hence the name.

And now take a look at the yellow field. If you entered the 5 there, it would torpedo both the pink and the green solution.
Consequently, the 5 cannot be a candidate there!

Sudoku example X-Wing

This is the same picture. There's an X-Wing . The 5 is again the candidate, but this time as seen from the column.

The 5 can be deleted in the yellow box.

The X-Wing is a special case of Multidimensional groups.

Sudoku example X-Wing

see also: Generalization : Multidimensional groups



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