The 12-sided polyhedral dice was always my favorite in the D&D set (d4, d6,d8, d12, d20), but it was also the least used. Why was it there in the first place?
It turns out, the D&D polyhedral dice set comprises the five Platonic Solids. Yes, "Platonic" referring to Plato himself, who postulated that the basic classical elements of the universe were made of these solid forms. Theaetetus, a contemporary of Plato, proved that these five shapes are the only regular convex polyhedral shapes that exist. "Regular" means essentially that if you roll the die, you have an equal probability of landing on any of the faces.
So why is the 12-sided dodecahedron dice there? Because otherwise the universe would be missing one of its elements! Certainly this applies to the D&D universe as well. And that is not all ... it turns out the d12 is also the first one which embodies the famous Golden Ratio. It's no coincidence that I have always been drawn to the d12, and now I know why. I hope we see a dodecahedron or two somewhere in BG III; I am sure it would have made Plato happy.