Ron Blond based in Edmonton in Canada has provided Java applets that illustrate some of the ‘circle theorems’. These facts about lines in circles and the relationships between the angles in a circle always cause trouble when teaching GCSE classes. I suspect that being able to drag lines and points around to explain what changes and what stays the same may be useful.

• Angle at centre and circumference
• Angle between a tangent and a chord

The geometry applet may allow the same kind of demonstration to be built with thicker lines making it more suitable for projector use.