When I worked as a camera guy on film sets, this was a typical occurrence. You level out the camera trypod with the magic eye on the tripod. The magic eye being a small amount of liquid with a single bubble inside, pointing always upward.
Soon you realize that an surprising amount of walls are just not straight or level.
aka spirit level?
On that subject a laser level is indispensable for any work that needs to be straight. I don’t know how I managed without one before.