http://en.wikipedia.org/wiki/Pentagonal_trapezohedron
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Pentagonal trapezohedron
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| Pentagonal trapezohedron | |
|---|---|
 Click on picture for large version. | |
| Type | trapezohedra |
| Faces | 10 kites |
| Edges | 20 |
| Vertices | 12 |
| Face configuration | 5,3,3,3 |
| Symmetry group | D5d |
| Dual polyhedron | pentagonal antiprism |
| Properties | convex, face-transitive |
The pentagonal trapezohedron or deltohedron is the third in an infinite series of face-transitive polyhedra which are dual polyhedra to the antiprisms. It has ten faces (i.e., it is a decahedron) which are congruent kites.
It can be decomposed into two pentagonal pyramids and a pentagonal antiprism in the middle.
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[edit] As a die
During the seventies the first role-playing games and miniature wargames that used percentile-based skills (RuneQuest was the first of them) first used a couple of twenty-sided dice to get random decimal numbers, such as percentages. To do that the players used to roll two twenty-sided dice and then divided the results as if the dice were ten-sided. The typically pentagonal-trapezohedrical ten-sided dice were invented in the late seventies on this purpose and their invention was announced during the 1980 Gen Con.[1]
To improve rolling, the edges are usually rounded or sub-faces introduced by truncation. Each face has two long edges and two short edges. The five odd-numbered faces meet at the common vertex of their long edges. The five even-numbered faces meet at the common vertex of their long edges.
There seems to be a standard configuration for the numbers on 10-sided dice. If one holds such a die between one's fingers at two of the vertices such that the even numbers are on top, and reads the numbers from left to right in a zigzag pattern, the sequence obtained is 0, 7, 4, 1, 6, 9, 2, 5, 8, 3, and back to 0. (In this position, odd numbers appear upside-down.) Opposite sides on such a die total nine.
These dice are often sold in pairs for use as a percentile die. One die will signify tens from 00 through 90, and the other units from 0 to 9. The use of such markings is to generate random numbers from 00 to 99, also known as percentile.
Regular icosahedra with two sides each marked 0 to 9 are also referred to as ten-sided dice, and sometimes preferred due to their more regular shape (see platonic solid) that improves rolling.
[edit] References
- ^ Greg Peterson about Gen Con 1980: The big news of the year was that someone had 'invented' the ten-sided die.
[edit] Sources
- Cundy H.M and Rollett, A.P. Mathematical models, 2nd Edn. Oxford University Press (1961), (3rd edition 1989) p. 117
[edit] External links
- Weisstein, Eric W., "Trapezohedron" from MathWorld.
- Virtual Reality Polyhedra www.georgehart.com: The Encyclopedia of Polyhedra
- VRML model
- Conway Notation for Polyhedra Try: "dA5"
- Dungeons & Dragons Dice Roller
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