An interactive applet and associated web page that demonstrate the concept of complementary angles (angles that add to 90°). The applet shows two angles. You can drag the endpoints of each angle and the other angle changes so that they always add to 90°. They are drawn in such a way that it is visually obvious that together they form a right angle, although they are separate on the page. The angle measure readouts can be turned off for class discussions.

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.