Origami: Reutersvärd Triangle

Reutersvärd Triangle, designed by me

Many years ago, I designed the origami Arrow Illusion–an arrow that points in the opposite direction when viewed in a mirror.  It’s probably the biggest “hit” among my designs.  Recently, CFC (an Origami community) had a monthly challenge to create optical illusions, and it featured Arrow Illusion as an example.  So, I thought I’d design another.

I went through several ideas, but ultimately settled on the Reutersvärd Triangle.  It’s similar to the better-known Penrose triangle, but was actually created independently by Oscar Reutersvärd in 1934–before the Penrose triangle.

Like the Penrose triangle, the Reutersvärd Triangle is just an illustration–it’s impossible to actually make something shaped that way.  And yet, you can find many very convincing 3D-printed implementations of the Reutersvärd Triangle.  I felt this was a sign that I needed to origamize it.  (Note that the Arrow Illusion, too, was inspired by 3D printing!)

Actually designing the thing was fairly challenging, and I went through over half a dozen prototypes.  Even once I got the basic form down, I tried many ideas to optimize for simplicity and stability.  The end result is so elegant that its instructions fit on a single page.  Note: if you try to fold this, I suggest trying the “easy” version first, and make one with just 6 pieces of paper. It will look like this:

How does the illusion work?  The cubes are actually inverted.  Although it pretends to be a collection of cubes pointing outwards towards the camera, they’re actually internal corners pointing inwards.  It’s… not very convincing when you have the physical object right in front of you.  It’s most convincing when you take a photo in ambiguous lighting, and then sometimes it helps to turn the photo upside-down.  The lighting of the cubes doesn’t match the shadows behind the model, which may have caused some of you to see through the illusion already.  But in case you don’t see it, here’s a photo from the side: