In terms of movie ratings, X rating was the classification devised by the Motion Picture Association of America (MPAA) and the National Association of Theater Owners (NATO) in 1968 to designate certain films containing excessive violence or explicit sexuality. It was replaced in 1990 by the NC-17 rating (no one 17 and under admitted).
In the 1970s the X rating concept was used by the producers and exhibitors of pornographic movies as a promotional device. Though these films were not MPAA productions and the producers could not submit their films for review, the X rating was not trademarked by MPAA. This meant that pornographic films could be advertised as X-rated or XXX-rated, which suggested that MPAA's X rating was a code for hardcore pornography.
Film Rating System:
G: GENERAL AUDIENCES: All ages admitted.
PG: PARENTAL GUIDANCE SUGGESTED: Some material may not be suitable for children.
PG-13: PARENTS STRONGLY CAUTIONED: Some material may be inappropriate for children under 13.
R: RESTRICTED: Under 17 requires accompanying parent or adult guardian.
NC-17: NO ONE 17 AND UNDER ADMITTED.
Further information on how to read film ratings, how they were established and how you can get your film rated can be found at FilmRatings.com