English: Diagram showing the construction of a 2-lobed cycloidal rotor, like that found in a Roots blower. The red curve is an epicycloid (outside the larger generating circle with diameter D, black) and the blue curve is a hypocycloid. The smaller circles are 1/4 the diameter of the larger circle. The solid curve is the profile of the rotor - half is the epicycloid and half is the hypocycloid.