# Name this figure! **URL:** \ **Category:** Factual Questions\ **Created:** [August 30, 2005, 6:01pm UTC](https://boards.straightdope.com/t/name-this-figure/319291 "2005-08-30T18:01:15Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Strinka](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strinka/32/512_2.png) [@Strinka](https://boards.straightdope.com/u/Strinka)\ **Post date:** [August 30, 2005, 6:01pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/1 "2005-08-30T18:01:15Z")
What is the name for a regular polyhedron with twenty triangular faces? ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [August 30, 2005, 6:05pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/2 "2005-08-30T18:05:52Z")
dodecahedron ---
**Author:** ![capybara](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/capybara/32/202_2.png) [@capybara](https://boards.straightdope.com/u/capybara)\ **Post date:** [August 30, 2005, 6:08pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/3 "2005-08-30T18:08:01Z")
Are you sure it’s not an icosahedron? Isn’t a dodecahedron 12-sided? ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [August 30, 2005, 6:10pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/4 "2005-08-30T18:10:00Z")
> [@capybara](#): > > Are you sure it’s not an icosahedron? Isn’t a dodecahedron 12-sided? dammit! ---
**Author:** ![Ponder\_Stibbons](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponder_stibbons/32/343_2.png) [@Ponder\_Stibbons](https://boards.straightdope.com/u/Ponder_Stibbons)\ **Post date:** [August 30, 2005, 6:11pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/5 "2005-08-30T18:11:48Z")
_dodecahedron._ A polyhedron with 12 faces. _icosahedron._ A polyhedron having 20 faces. – Reference: [dictionary.com](http://dictionary.reference.com) ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [August 30, 2005, 6:13pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/6 "2005-08-30T18:13:34Z")
> [@Ponder Stibbons](#): > > _dodecahedron._ A polyhedron with 12 faces. > > _icosahedron._ A polyhedron having 20 faces. > > – Reference: [dictionary.com](http://dictionary.reference.com) Yeah, yeah I screwed up. However, technically the OP is looking for a _regular_ icosahedron, unless there is another word that means specifically that. ---
**Author:** ![glee](https://avatars.discourse-cdn.com/v4/letter/g/b5a626/32.png) [@glee](https://boards.straightdope.com/u/glee)\ **Post date:** [August 30, 2005, 6:14pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/7 "2005-08-30T18:14:06Z")
As a roleplayer, this is right up my street. I use the following: 4-sided pyramid 6-sided cube 8-sided octogon 12-sided dodecahedron 20-sided icosohedron (I may have muddled the names in naming solid versions, but I’m certain my prefixes are correct. 😕 ) ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [August 30, 2005, 6:15pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/8 "2005-08-30T18:15:26Z")
Try octohedron and tetrahedron for the 4- and 8-sided dice, respectively. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [August 30, 2005, 6:21pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/9 "2005-08-30T18:21:23Z")
Other way around, ultrafilter. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 30, 2005, 6:27pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/10 "2005-08-30T18:27:21Z")
Although a tetrahedron is a type of pyramid, specifically an equilateral triangular-based pyramid. Also note that the terms “tetrahedron”, “icosohedron”, etc. don’t necessarily imply any degree of regularity, just a number of faces. If, for instance, you take a cube and lop off two of the corners, the resulting figure will be an octahedron, just a very irregular one. Similarly, a ten-sided die could properly be called a decahedron (note no do- at the beginning). ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [August 30, 2005, 6:38pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/11 "2005-08-30T18:38:17Z")
Here are some [pretty pictures](http://mathworld.wolfram.com/PlatonicSolid.html) and interactive models of all five Platonic solids, including the regular icosahedron. If you don’t care about the triangular faces being identical (the OP doesn’t specify), then there’s also [Jessen’s orthogonal icosahedron](http://mathworld.wolfram.com/JessensOrthogonalIcosahedron.html), which still counts as a regular polyhedron. They make lousy dice however. ---
**Author:** ![wolf\_meister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolf_meister/32/15202_2.png) [@wolf\_meister](https://boards.straightdope.com/u/wolf_meister)\ **Post date:** [August 30, 2005, 7:44pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/12 "2005-08-30T19:44:31Z")
**Bytegeist** I don’t believe Jessen’s orthogonal icosahedron is a regular polyhedron. I also believe that there are only 5 regular polyhedra. (The ones listed by **glee** and then corrected by **Chronos** ). ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [August 30, 2005, 7:56pm UTC](https://boards.straightdope.com/t/name-this-figure/319291/13 "2005-08-30T19:56:16Z")
> [@wolf\_meister](#): > > **Bytegeist** > I don’t believe Jessen’s orthogonal icosahedron is a regular polyhedron. > I also believe that there are only 5 regular polyhedra. (The ones listed by **glee** and then corrected by **Chronos** ). Ah, you’re right — in the first part certainly. Although, according to [here](http://mathworld.wolfram.com/RegularPolyhedron.html), the term “regular polyhedron” can sometimes include [these four](http://mathworld.wolfram.com/Kepler-PoinsotSolid.html) concave polyhedra. None of them has 20 sides though. In any case, the Jessen’s orthogonal icosahedron doesn’t fit either sense of the term, so I stand corrected.