An epitrochoid is traced by a point attached to a circle of radius

rolling around the outside of a fixed circle of radius

, where the point is at a distance

from the center of the exterior circle. The Spirograph™ toy, using toothed gears, is constrained to a set of rational values for

, giving closed curves. Here,

and

is constrained to rational values

. Animating the parameter

portrays the familiar drawing mechanism.