Five tetrahedra fit neatly into the corners of a dodecahedron. See http://www.shapeways.com/model/121250/dodecahedron___5_tetrahed.html by LatexSpikes for a beautiful wirefram example. So, when rolling the compound, it'll always land on five points, like a dodecahedron, and so we can number it like a d12. There are actually two chiral forms of this shape, and this is one of them. Combining both yields another, more symmetrical shape.