[R] rgl terminology: the definitions of "thickness" and "width" in rgl

[Byron Dom]

I'm struggling to come up to speed with rgl. At the moment, I'm struggling with the function arrows3d(). In the R documentation for that, two parameters/arguments are mentioned: "thickness" (of the arrow's shaft) and "width" (of the arrow's shaft). 
Naively, I would expect the arrow's shaft under normal circumstances to be cylindrical and its width or thickness would reduce to the single parameter "diameter." Then in some cases like plotting a 2-dimensional arrow on a 2-dimensional surface embedded in a 3D space, things like "width" would have an obvious meaning.
Would someone explain this to me, including the difference between width and thickness.
Thanks in advance.

	[[alternative HTML version deleted]]