If two pairs of opposite polygon vertices of a complete quadrilateral are pairs of harmonic conjugate points, then the third pair of opposite polygon vertices is likewise a pair of harmonic conjugate points.
Hesse's Theorem
See also
Complete QuadrilateralExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Hesse's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HessesTheorem.html