An interactive applet and associated web page that demonstrate the concept of a concave polygon - one where at least one interior angle is greater than 180°. The applet shows an irregular polygon initially with one interior angle greater than 180°. The user can drag any vertex and change the number of sides in the range 3..99. When the polygon is concave, the angles that make it so are drawn in red. The goal is to show through experimentation what the concept of concavity really means.

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.