I once read that there's no "XXX" MPAA rating. It was actually a porn industry marketing "rating" that came out after "Midnight Cowboy" was released with an "X" rating, and the porn industry didn't want people getting "confused." Besides, they figured if an "X" rating would be a dirty movie, "XXX" would be REALLY dirty! LOL
Mark
It all happened because for reasons no one can adequately explain the MPAA didn’t bother to trademark the X rating. G, PG and R (and eventually PG-13) were all trademarked, X they didn’t, so the porn industry leveraged it by labeling their stuff XXX (like X only more). Eventually the MPAA fixed this by replacing X with the trademarked NC-17, which we all still call X.