Gallatly, W. "Orthologic Triangles." §82 in The
Modern Geometry of the Triangle, 2nd ed. London, England: Hodgson, pp. 55-56,
1913.Pedoe, D. "On (What Should Be) a Well-Known Theorem in Geometry."
Amer. Math. Monthly74, 839-841, 1967.Steiner, J. "Vorgelegte
Lehrsätze." J. Reine Angew. Math.2, 287-292, 1827.
and 
are orthologic if the perpendiculars
from the vertices 
, 
, 
on the sides 
, 
, and 
are concurrent. Furthermore,
if this is the case, then the perpendiculars from
the vertices 
, 
, 
on the sides 
, 
, and 
are also concurrent, as
shown by Steiner (1827). This reciprocal property is the perpendicular form of Maxwell's theorem (Pedoe 1967).
with respect to 
.
