points on a line l are harmonically related if the line is determined
by a pair
of points from the intersection of lines in a complete quadrangle and
that line with the other two sides of the complete quadrangle.
In the figure: See Test Java Applet
The line AB determines two other points, XZ∩AB=C and YW ∩ AB=D, so that the points ABC and D are "harmonically related."This
is denoted H(AB,CD).