Truncating edges down to points produces the octahedron as a rectified tetrahedron. The tetrahedron, cube, and octahedron all occur as crystals. The octahedron is also a "rectified tetrahedron". In Conway polyhedron notation it can be seen as gT, a gyro tetrahedron. A truncation process applied to the tetrahedron produces a series of uniform polyhedra. They correspond to the double cover of the rotational symmetry group of the regular tetrahedron. The hydrogen atoms are close to two corners of a tetrahedron centered on the oxygen. The regular tetrahedron is the simplest Platonic solid. These seven line segments all meet at the "centroid" of the tetrahedron. A tetrahedron having stiff edges is inherently rigid. An equivalent description describes the tetramer as a Li tetrahedron interpenetrated with a tetrahedron ["C"HR]. One such regular tetrahedron has a volume of of that of the cube. A similar approach allows one to deduce the equation of the circumsphere of a tetrahedron. See also the isometries of the regular tetrahedron. Each F centre is coordinated to four Ca centres in the shape of a tetrahedron.