**Definition:
Four
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
the ****intersection
of
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).**