This Demonstration gives an animation of an epitrochoid and associated planetarymotion envelope curve The configuration shown has applications in the internal combustion engines invented by Felix Wankel and popularized by Mazda in RX7 and RX8 cars The eccentricity ratio changes the shapes of the curves The reference frame determines what is held stationary in the animation either the epitrochoid blue the envelope curve purple or the centers of rotation of both curves The inner envelope is the triangular rotor shape used in place of a piston in a Wankel rotary engine whereas the outer envelope is the continuation of the envelope curve along the opposite extreme of motion