About the twelve-sided die
The d12 is a dodecahedron: twelve regular pentagons, three meeting at each of its twenty corners. It is one of the five Platonic solids, rolls smoothly, and its opposite faces add up to 13. Each number has a 1-in-12 chance (8.33%), and the average roll is 6.5.
What the d12 is used for
In D&D the d12 is the big hit: the greataxe and lance deal 1d12, and barbarians — the toughest class — use a d12 hit die. It is often joked to be the least-rolled die in the set, which only makes the moment you need it sweeter.
Outside games, twelve sides are handy for anything that comes in twelves: pick a random month, an hour on a clock face, or a sign of the zodiac.
1d12 is not the same as 2d6
Both top out at 12, but they behave very differently. A d12 is flat — a 1 is as likely as a 6. Two d6s can never roll a 1 and pile up around 7. Choose 1d12 when every result should be equally likely, and 2d6 when you want the middle to be common.
| Result | 1d12 | 2d6 |
|---|---|---|
| 1 | 8.33% | 0% |
| 2 | 8.33% | 2.78% |
| 3 | 8.33% | 5.56% |
| 4 | 8.33% | 8.33% |
| 5 | 8.33% | 11.1% |
| 6 | 8.33% | 13.9% |
| 7 | 8.33% | 16.7% |
| 8 | 8.33% | 13.9% |
| 9 | 8.33% | 11.1% |
| 10 | 8.33% | 8.33% |
| 11 | 8.33% | 5.56% |
| 12 | 8.33% | 2.78% |
A curious cousin
More than a hundred hollow bronze dodecahedra survive from the Roman provinces, mostly from the 2nd to 4th centuries. They are not dice — each face has a round hole of a different size — and nobody is sure what they were for. Theories range from knitting tools to surveying instruments.