Now that we've discussed exploring the entirety of the three dimensions available to us, lets look at the objects that can exist within space - more specifically, their surfaces.
Contour lines are the marks that run along the surface of an object. In doing so, they provide our eyes and brain with valuable information describing just how those surfaces themselves flow through space.
For example, let's imagine that inside of our 3D world, there exists a piece of paper, and on that piece of paper, there has been made a single straight line down its length. The paper is flat and straight, so the line still reads as visually being straight. But what if we were to take that piece of paper and rolled it up?
Now that line as we see it would be curved, wrapping faithfully around the cylinder we've created and describing to us the transformation that piece of paper has undertaken, from being flat to cylindrical. That is what a contour line does.
Not only does it provide the viewer's brain with additional information to help understand what it is looking at, but it also helps us as we're drawing to better understand how these forms and surfaces exist in three dimensional space. That ultimately comes back to helping us believe in the lie we're trying to create.
This also means that every single mark that exists on a surface can also work against us. We are in full control of which marks go where, so we could absolutely draw a line that appears straight on the surface of a cylinder, and it would immediately ruin the illusion by making the cylinder read as flat.