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).

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