The mesh implemented in this chapter may be viewed as a generalization of the mesh implemented in the previous chapter to three spatial dimensions. Indeed, it is implemented as a linked list of tetrahedra, each of which may be viewed as the three-dimensional generalization of a triangle. Furthermore, the iterative refinement process preserves conformity: once a particular tetrahedron is refined by dividing one of its edges and connecting its midpoint to the corner that lies across from it to form two subtetrahedra, all the neighbor tetrahedra that share this edge are also refined in the same way, using the midpoint as a new corner in the new subtetrahedra.