Interior wall how to get it straight?

[zana kader]

Hello, I'm going to build an interior wall. How do you know that the wall from side to side is straight?

 

[KnockOnWood]

Which one is supposed to be straight, do you mean?
The wall goes from one wall to the other, are those walls "straight"?
But otherwise, it's probably a level or plumb line that's needed to build a small wall straight.

 

[BerraBo]

Assuming the OP means how the wall should be straight in relation to the other walls, i.e., 90 degrees.
If it's not too far to another parallel wall, you can measure the same distance on both sides.
Otherwise, it's common to use Pythagoras' theorem and measure a triangle with the ratios 3:4:5 where 5 is the diagonal.