Fine line marker. Pencil and ruler for construction lines, also lots of math.
This one was made for my math methods class when I was still studying to be a math teacher. Our teacher decided to give us a math & arts assignment and I thought that was the coolest idea so I went with it full steam. I used a shape inspired by a tweet by Dan Meyer about a kids' fun math book, and mixed it with my usual celtic knot style. I had to use some algebra (you know, your casual system of three equations with three unknowns) to ensure the two threads would be perfectly aligned. So, yeah, a very mathy creation.