Two Dürer's Foliums
Dürer's folium is a special epitrochoid. This Demonstration shows how two such curves, with one of them rotated by 90°, are defined by rolling two small circles of radius over a larger circle of radius .[more]
The "phase" control draws the curves; the "transparency" control shows the three points and circles.
The folium is also the inverse of Delanges's trisector, and thus can be used to trisect angles.[less]