The principle of heavy vertical line or horizontal line is

As shown in the figure, the heavy vertical level is mainly composed of vertical base, bracket and pendulum, and some brackets have angles.

Because the level is placed on a plane, the vertical line of the traction pendulum is along the direction of the heavy vertical line, and its angle with the bracket is equal to the angle between the base and the horizontal plane (there should be many such proof questions in junior high school, so I won't explain them here). Some can be read directly, and some need to be measured with a goniometer. This method can easily measure the angle between the plane and the horizontal plane.