The planes passing through the vertices of a tetrahedron and tangent to the circumsphere
at these points form another tetrahedron called the tangential tetrahedron.
The four lines of intersection of the faces of a tetrahedron with the corresponding faces of its tangential tetrahedron form a hyperbolic group (Altshiller-Court 1979, p. 102).
and tangent to the circumsphere
at these points form another tetrahedron called the tangential tetrahedron.
