You can use a few tools to measure the straightness of a wall, depending on the level of precision you need:

For basic straightness:

* Level: A level is the most basic tool and will show if a wall is vertically or horizontally straight.

* Plumb bob: A plumb bob can be used to determine if a wall is perfectly vertical.

For more accurate measurements:

* Laser level: A laser level projects a horizontal or vertical line, making it easy to see if a wall is straight.

* Straight edge: A long, straight piece of wood or metal can be used to check the straightness of a wall over a longer distance. You can use a level to ensure the straight edge itself is level.

For highly precise measurements:

* Total station: A total station is a surveying instrument that uses lasers and prisms to measure distances and angles with extreme accuracy.

The best tool for measuring the straightness of a wall depends on the specific application and the required level of precision.