Read the following text:
The toolbox in a carpentry workshop usually contains various tools such as a hammer, a tape measure, and a level. The hammer is used for pounding nails into wood, while the tape measure is used to measure the length, width, and height of a wooden object. The level, on the other hand, is used to ensure that the surface of the object is perfectly horizontal or vertical.
What is the primary function of a level in a carpentry workshop?