So, this is like a pen of sorts? It's like a metal quill. You stick an end into some fountain pen, and off you go. No, seriously. You can adjust the thickness. It also can bend and detach (I think it's supposed to be used with a compass?)
It's really super at straight lines. Curves I'm getting used to.
I'm in utter disbelief that this works at all.
Apparently this is called a ruling pen: https://en.m.wikipedia.org/wiki/Ruling_pen