Double Letters

I saw several examples of 3D letters used to produce 2 different words depending on the side from which you see them. I tried to check the web to find more ideas and realization but I don't know how to explain what I'm looking for.
The first time was a few years ago in an exhibition about optical illusion. The object in the middle of the room was just some lines with no meaning. But when you turn around suddenly you could read YES, and if you turn more on the other side it was written NO. Of co