If we add a height to the length and width of an area, we leave plane geometry and come to stereometry. In this area of mathematics we encounter four basic figures: Kugel, Kegel, cylinder and polyhedron. Polyhedra are bodies with length, width and height. Each face on it is a polygon. There are only five regular polyhedra.
A tetrahedron or pyramid has four faces; each represents an equilateral triangle.
A hexahedron or cube has 6 faces; each is a square.
has an octahedron 8 surfaces; all faces are equilateral triangles.
A dodecahedron has 12 surfaces; each is a pentagon.
Has an icosahedron 20 surfaces; all are equilateral triangles.