Fig. 536.03 Domains of Vertexes, Faces, and Edges of Systems: A. The domain of the vertex of a system: the domain of each vertex of the icosahedron is a pentagon whose edges connect the centers of gravity of five icosahedron face triangles. The resulting figure is the pentagonal dodecahedron. B. The domain of the face of a system: The domain of each face of the icosahedron is the triangular face itself. C. The domain of the edge of a system: The domain of each edge of the icosahedron is a diamond formed by connecting the vertexes of two adjacent icosahedron face triangles with their centers of gravity.