Designed by David E. Joyce, a math and computer science professor at Clark University, this site combines one of the most influential works of mathematics with modern technology to make math come alive. Joyce uses java applets to illustrate Euclid’s principles. By clicking on points in the diagrams, you can manipulate them to show the relationships between line segments, angles, shapes, and planes. Even if your browser doesn’t support java, you’ll appreciate the site’s comprehensive collection of definitions, propositions, and corollaries along with their illustrations.