In details

Polyhedra


We call it polyhedron the solid bounded by four or more flat polygons, belonging to different planes and having two by two only one edge in common. See some examples:



Polygons are the faces of polyhedron; the sides and vertices of polygons are the edges and vertices of the polyhedron.

Convex and concave polyhedra

Looking at the polyhedra above, we can see that considering any of its faces, the polyhedra are entirely in the same semi-space as this face determines. So these polyhedra are called convex.

This is not the case in the last polyhedron, because in relation to two of its faces it is not contained only in a semi-space. Therefore, it is termed concave.

Classification

Convex polyhedra have special names according to the number of faces, such as:

  • tetrahedron: four faces

  • pentahedron: five faces

  • hexahedron: six faces

  • heptahedron: seven faces

  • octahedron: eight faces

  • icosahedron: twenty faces

Next: Regular Polyhedra