This is a trick dovetail joint. When assembled it looks impossible, but it's trivially simple to open.
I've searched online and found lots of other people have independently invented this mechanism, probably going back over a century. Some designs have added latches and machinery, but some details are universal!
If you build the parts in two different materials then build Dovetail Triangle1.stl in the darker, more opaque material for the best-looking joint.
This should build without needing support, as the maximum overhang is less than 45-degrees even though it looks steeper. The pictured sample was built in PVC on my Solido SD300.
More pictures on my blog at http://mysd300.blogspot.com/2011/09/triangular-dovetail-joint.html