An interactive applet and associated web page that demonstrate the interior angles that are formed where a transversal crosses two lines. The applets shows the two possible pairs of angles in turn when in animation mode. By dragging the three lines, it can be seen that the angles are supplementary only when the lines are parallel. When not in animated mode, there is a button that alternates the two pairs of angles.

The text on the page discusses the properties of the angle pairs both in the parallel and non-parallel cases.

Applet can be enlarged to full screen size for use with a classroom projector. After use in the classroom, student can access it again from any web browser at home or in the library with no login required.

This resource is a component of the Math Open Reference Interactive Geometry textbook project at http://www.mathopenref.com.