This is a test of commonsense. Complete the next sentence:

How to measure parallelism
Define the surface for testing.
Parallelism is a quality that can be tested on several different types of surfaces. These are labeled based on their positions relative to each other.
You may wish to test parallelism of the following : Planar surface: opposite flat surfaces of a rectangular solid need to be parallel to each other. Cylindrical surface: a cylinder appears round when looking at its end, but when held upright, its sides must be parallel for the cylinder to function properly.