India ink on Bristol paper, 18x18" (frame not included)
A trefoil knot is the simplest non-trivial knot.
As usual, this drawing was created programmatically, but required an unusual number of separate programs. First, I wrote code to generate a 3D model of a trefoil knot. Then, I used a homegrown ray tracer to compute the shading (which you can see in the "ray-traced reference image" above). Then, a third program generated the vector lines, modulated by the shading levels. Finally, it was drawn with a pen plotter.
I personally fulfill these orders on demand. They are drawn with an AxiDraw V3/A3 pen plotter using a Koh-I-Noor Rapidograph technical pen on Strathmore Smooth Bristol paper. I number and initial each drawing by hand.