> It's key that the writing point is at a 90 degree angle to the device spines But the device in the picture is exactly like that one! The three main spines wi

> It's key that the writing point is at a 90 degree angle to the device spines But the device in the picture is exactly like that one! The three main spines will always occupy exactly the same positions in either the device in the picture or in the device in the video; only the interior hinges used to enforce it are different. The device lies flat on the paper in both cases, and the writing point (your pencil or whatever you stick in the holes) will of course be at 90° angle to the device spines. Actually a better search found names for these devices: see terms like “Sylvester's Link Fan” and Laisant's Link #2 here: http://www.takayaiwamoto.com/Greek_Math/Trisect/Linkage/Link_Laisant.html http://www.takayaiwamoto.com/Greek_Math/Trisect/Linkage/Link... — and a much better video showing almost exactly the device in the picture: https://www.youtube.com/watch?v=uTQVi7zskGU https://www.youtube.com/watch?v=uTQVi7zskGU (except for quadrisection instead of trisection). (And the article does mention the Laisant reference; hadn't noticed it earlier: see p.38 here: https://files.eric.ed.gov/fulltext/ED058058.pdf#page=49 https://files.eric.ed.gov/fulltext/ED058058.pdf#page=49 ) > [...] straightedge with a fixed pair of marks [...] origami Fair enough; these devices are indeed simpler to construct, but the actual process of trisection of a given angle is not as simple as “plonk the device on the paper, open it up to the angle, and there, it's already trisected”. Nor is the proof that you've successfully trisected as straightforward.