I didn't accurately represent ScreenSpaceLines3D when I called the class a kind of "line primitive." Unlike a primitive, the class, when rendered, will create a line that has a constant thickness in screen-space. In this way, it can be reliably seen regardless of its depth. This constant thickness is a different effect than if you were to take a cube and scale it along a single axis. The WPF engine is doing work to create this illusion. Thus ScreenSpaceLines3D lines are also known as magic lines. Check them out!