For those of you just dying to know, the peritrochoid is the basic curve that forms the inner surface of a Wankel engine's rotor housing. In his book Rotary Engine, author Kenichi Yamamoto, godfather of the Mazda Rx-7, describes it thusly:

	It is the actual base for the geometric construction of the rotary engine. The peritrochoid is the locus of the tip point P of an arm fixed on the revolving circle B in Radius q when it rolls along the periphery of the base circle A in Radius p as inscribed.

RotaryRocket will take the liberty of assuming that at this point, his readers either have achieved full comprehension of the preceding concept, or no longer give a damn. :-)