A web page and interactive applet that illustrate the concept of locus.

The applet shows a model of the plane populated with "all" the possible points on the plane. Choosing to highlight those that are all a given distance from a given point, for example, results in a circle. The user can drag the point and change the distance, showing the circle moving and changing size.

The text page has a description of the locus concept and links to other loci - a straight line and an ellipse.

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.