A Perspective Collineation with center and axis not incident is called a Homology. A Homology is said to be harmonic if the points and on a line through are harmonic conjugates with respect to and . Every Perspective Collineation of period two is a harmonic homology.
See also Homology (Geometry), Perspective Collineation
Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, p. 248, 1969.