Planes are parallel to each other if they don't intersect. Planes are perpendicular to each other if one of the planes contains a line perpendicular to the other plane. Here are the drawings:
A polyhedron is a simple closed surface that is the union of polygons. (Sound familiar? Remember that a polygon is a simple closed curve that is the union of segments. Much of three-dimensional geometry is only an extension of two-dimensional, or planar, geometry.) A polyhedron encloses a region in space. It does so with parts of intersecting planes, each part being a polygon. Each polygon that composes a side of a polyhedron is called a face of the polyhedron. The intersection of two faces is called an edge. The intersection of three or more faces is called a vertex. Below are pictured examples of polyhedra. The following lessons will discuss certain kinds of polyhedra and their general forms.