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### Types of Pyramids

```
Date: 06/08/99 at 00:21:07
From: Rhiannon Lipman
Subject: Mathematical Pyramids

What are the types of pyramids? I know that a pyramid with a base that
has four equal sides is a square pyramid, but what are the other types
called and what do they look like?
```

```
Date: 06/08/99 at 01:29:58
From: Doctor Barrus
Subject: Re: Mathematical Pyramids

Hi, Rhiannon!

Good question! You're right - a pyramid with a base that is square is
called a square pyramid. Most other pyramids are called by the name of
their base. So, for example, a pyramid whose base has five equal sides
(in other words, whose base is a regular pentagon) is called a
pentagonal pyramid. A 6-sided (hexagon) base pyramid is called a
hexagonal pyramid, a 7-sided (heptagon) one is called a heptagonal
pyramid, etc.

There's one pretty important exception to that rule. It's the pyramid
whose base is an equilateral triangle - a triangle with 3 equal
sides. If the sides of the pyramid that go up from the base to the top
of the pyramid are all this same length, too, then the pyramid is
called a tetrahedron. This is an important pyramid. It has 6 edges,
all of the same length, and 4 sides that are all exactly the same
shape! It's been known for thousands of years.

For a picture of a tetrahedron, see the Dr. Math FAQ:

http://mathforum.org/dr.math/faq/formulas/faq.polyhedron.html#tetrahedron

or Peter T. Wang's Platonic Solids page:

http://www.ugcs.caltech.edu/~peterw/studies/platonic/tetrahedron.html

I hope this is clear and has helped. Good luck!

- Doctor Barrus, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Geometry
High School Polyhedra
Middle School Geometry
Middle School Polyhedra

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