For example, consider any two successive planes in a closest packed lattice. One atom in the A layer nestles in the triangular groove formed by three adjacent atoms in the B layer, and the four atoms touch along the edges of a regular tetrahedron. The edges of the tetrahedron are of length 2r, and the center of the tetrahedron is a cavity called the Tetrahedral hole (a common symbol for "tetrahedral" is Td). If a guest sphere in that cavity is in contact with all four host spheres, its radius is about 0.22r.
