© Islington Artefact Library

Pyramids are threedimensional (3D), unlike triangles, which are flat, or twodimensional (2D). This means they have depth as well as length and width. Pyramids have a polygon at their base (a polygon is a plane or flat shape with 3 or more sides). The name of a pyramid depends upon the shape of its base. For example, pyramids with a square at their base are known as 'square based' pyramids, such as the one in the picture and those in Egypt. A pyramid with a triangular base is called a 'tetrahedron'. The other faces are triangles, which meet at a vertex at the pyramids top.
Pyramids are members of the polyhedron family. A polyhedron is a solid shape which is made from lots of polygons. The word polyhedron comes from a Greek word meaning many faces. Polyhedra have flat faces, straight edges and vertices. There is a formula for connecting the number of faces, edges and vertices on a solid, made famous by the Swiss mathematician Euler (17071783). The formula is:
Vertices + Faces  Edges = 2
The height of a pyramid is the perpendicular distance from the top (vertex) to the plane of the base. The volume of a pyramid is onethird of the volume of a prism that has the same base and altitude. The equation for the volume of a pyramid is hK/3, in which h is the height of the pyramid and K is the area of the base.
The most famous pyramids of all were built in ancient Egypt between about 3200  2500 BC. They were the monuments and burial places for the royal families of Egypt  in particular their kings who were called Pharaohs. We may never know why the Egyptians chose the pyramid shape. It may have developed from earlier burial mounds, or been a symbol of the sun's rays or a stairway to heaven. Pyramids were also built in Central America by the Mayan people between AD 300 and 900, and later by the Aztecs. They were used to worship their sun, moon and rain gods.
