The circular points at infinity, also called the isotropic points, are the pair of (complex) points on the line at infinity through
which all circles pass. The circular points at infinity belong to the lines with
slopes 
and 
.
In the plane of a triangle, the circular points at infinity are isogonal
conjugates of each other.
All conics passing through the circular points at infinity are circles.
The circular points at infinity are the fixed points of the orthogonal involution.
Circular points at infinity were first considered by Poncelet in 1813 (Coxeter 1993).
