### X Wing

Take a look at row number 2 and row number 4 in this puzzle.

Let's see which cells can contain the digit 8 in those two rows, they are highlighted in red.

It happens to be that those 4 cells (2 in each row) align up perfectly in the same two columns - column 5 and column 7.

This is called an **X Wing**.

These 4 cells form a shape of an **X** or a rectangle.

Rows number 2 and 4 are called **base sets** and the digit 8 is called **fish digit**.

There are more possible 8's in columns 5 and 7.

They highlighted in grey and we'll get back to them later.

Let's look back at rows R2 and R4 - there are only two possible ways for 8 to reside in those two rows.

Both are in opposite corners of the X Wing.

Option 1: If R2C5 is an 8 then neither R4C5 nor R2C7 can be an 8 (basic rule of Sudoku).

Therefor R4C7 must be an 8.

Option 2 is the other way round.

If R4C5 Is 8 than R2C7 must also be an 8.

Either way, if we look at column 5 it must have an 8 on R2C5 or R4C5.

Therefor we can eliminate 8 from all other cells in this column - R5C5 in particular.

Same thing with column 7 - it must have an 8 on R2C7 or R4C7.

Therefor we can eliminate 8 from all other cells in this column - R3C7.

This is what **X Wing** is about, eliminating candidates using logic.

Here is another example of X Wing.

Rows 6 and 9 are the **base sets**.

3 is the **fish digit**.

The possible cells for 3 in these rows are aligned up perfectly in the same two columns - column 4 and column 8.

They form an X Wing.

We can therefore eliminate 3 from all other cells in those 2 columns as you can see in the picture.

X Wing can also appear where the base sets are in column.

In the following example - columns 5 and 7 are the **base sets**.

3 is the **fish digit**.

The possible cells for 3 in these columns are aligned up perfectly in the same two rows - row 2 and row 7.

They form an X Wing.

We can therefore eliminate 3 from all other cells in those 2 rows as you can see in the picture:

**How to find X Wings**

Locating x wing is not very difficult once you have pencil marks in your Sudoku.

You should scan rows and columns for each digit and find a set of two rows or columns that align-up perfectly together.

Can you find the X Wing with the base of 8 in this puzzle?

The base rows are R4 and R9.

They are the only possible cells for 8 in rows 4 and 9 and they align up in the same columns.