Illustration of two examples (helices, and loops) of technique for making hand-drawn ribbon diagrams.  [This is two photographs of 1984 pen-and-ink hand drawings, by Jane Richardson.]
<a href="//commons.wikimedia.org/wiki/User:Dcrjsr" title="User:Dcrjsr">Dcrjsr</a>
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>