Varignon's theorem states that the figure formed when the midpoints of the sides of a convex quadrilateral are joined
in order is a parallelogram. Equivalently, the
bimedians bisect each other.
Coxeter, H. S. M. and Greitzer, S. L. "Quadrangles; Varignon's Theorem." §3.1 in Geometry
Revisited. Washington, DC: Math. Assoc. Amer., pp. 51-56, 1967.