The six planes through the midpoints of the edges of a tetrahedron and perpendicular to the opposite edges concur in a point known as the Monge point.
Monge's Tetrahedron Theorem
See also
Monge Point, Plane, TetrahedronExplore with Wolfram|Alpha
References
Altshiller-Court, N. "The Monge Theorem." §228 in Modern Pure Solid Geometry. New York: Chelsea, p. 69, 1979.Forder, H. G. Math. Gaz. 15, p. 470, 1930-1931.Lez, H. and Dugrais, M. "Solution des questions proposées dans les Nouvelles Annales: Question 906." Nouvelles ann. de math. 8, 173, 1869.Monge, G. Corresp. sur l'École Polytech. 2, 266, 1795.Thompson, H. F. "A Geometrical Proof of a Theorem Connected with the Tetrahedron." Proc. Edinburgh Math. Soc. 17, 51-53, 1908-1909.Referenced on Wolfram|Alpha
Monge's Tetrahedron TheoremCite this as:
Weisstein, Eric W. "Monge's Tetrahedron Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/MongesTetrahedronTheorem.html