Two triangles 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 in 1827.
The point of concurrence is known as the orthology center of
with respect to .
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 in 1827.
with respect to 
.
