Fig. 1032.12 Convex and Concave Sphere Packing Voids: A. In a tetrahedron composed of four spheres, the central void is an octahedron with four concave spherical triangular faces and four planar triangular faces with concave sides. This can be described as a “concave octahedron.” B. In an octahedron composed of six close-packed spheres, the central void is a vector equilibrium with six concave spherical square faces and eight triangular faces with concave sides: a “concave vector equilibrium.” C. The vector equilibrium with edges arced to form a sphere: a convex vector equilibrium. D. The vector equilibrium with arcs on the triangular faces defined by spheres tangent at vertexes: a concave vector equilibrium.