The orthopoles of a line with respect to the four triangles formed by three out of
four vertices of any quadrilateral lie on a straight line known as the orthopolar line of for the given quadrilateral
(Servais 1923, McBrien 1942, Goormaghtigh 1947).
Goormaghtigh, R. "Orthopolar and Isopolar Lines in the Cyclic Quadrilateral." Amer. Math. Monthly54, 211-214, 1947.McBrien,
V. O. On Some Systems of Orthopolar and Kantor Lines of Lines Referred to
a Quadrangle. Washington, DC: Catholic University of America Press, 1942.Servais,
C. "Sur l'orthopóle." Mathesis, p. 11, 1923.
with respect to the four triangles formed by three out of
four vertices of any quadrilateral 
lie on a straight line 
known as the orthopolar line of 
for the given quadrilateral
(Servais 1923, McBrien 1942, Goormaghtigh 1947).
