From Sudopedia, the free Sudoku reference guide

From Sudopedia

An X-Wing is a fish pattern involving 2 rows and 2 columns.

It is a single-digit solving technique that allows the player to eliminate candidates.

It is an example of a simple X-Cycle (continuous X-Cycle with length of four) or a Nice Loop Rule 1 (candidates eliminated outside the loop but within the unit of the links with weak inference).

Example

Try to find an X-Wing; scroll down to see the Solution.

It is in rows 1 and 4.

Make sure that both rows each contain only 2 candidates of a given digit, and that these candidates are confined to two columns. In this example, the columns are 2 and 5.

Now we can eliminate the remaining candidates for that digit from these two columns.

A total of 9 candidates can be eliminated from these two columns.

How it works

When 2 rows each contain only 2 candidates for a given digit, and the candidates are located in the same two columns, you may eliminate all other candidates for that digit from these 2 columns.

Example. fish diagram:

There are 2 candidates in row 2 and in row 8, marked X. Both candidates are located in the same columns 2 and 8. All other candidates in these 2 columns, marked with a red asterisk, can be eliminated.

Clearly, the same logic can be applied when 2 candidates for a given digit appear exactly twice in 2 columns and they are in the same 2 rows. Now the other candidates for that digit can be removed from these two rows.