In some countries, X or XXX is or has been a motion picture rating reserved for the most explicit films.
For example, in the United States, the X rating originally referred to a non-trademarked rating that indicated a film contained content that was not suitable for children, such as extreme violence, and thus was for adults only. When the MPAA film rating system began on November 1, 1968 in the U.S., the X rating was given to a film by the MPAA if submitted to them or, due to its non-trademarked status, it could be self-applied to a film by a distributor who knew beforehand that their film contained content unsuitable for minors. In the late 1960s to mid-1980s, several mainstream films were released with an X rating such as Midnight Cowboy (won Best Picture, Best Director, and Best Writing at the 1970 Oscars), A Clockwork Orange, Fritz the Cat, Last Tango in Paris and The Evil Dead. Because the X rating was not trademarked, anybody could apply it to their films, including pornographers, which many began to do in the 1970s. As pornography began to become chic and more legally tolerated, pornographers placed an X rating on their films to emphasize the adult nature of them. Some even started using multiple X’s (i.e. XX, XXX, etc.) to give the impression that their film contained more graphic sexual content than the simple X rating. In some cases, the X ratings were applied by reviewers or film scholars, e.g. William Rotsler, who wrote “The XXX-rating means hard-core, the XX-rating is for simulation, and an X-rating is for comparatively cool films.” Nothing beyond the simple X-rating has ever been officially recognized by the MPAA. Because of the heavy use of the X rating by pornographers, it became associated largely with pornographic films, so that non-pornographic films given an X rating would have fewer theaters willing to book them and fewer venues for advertising. This led to a number of films being released unrated sometimes with a warning that the film contained content for adults only. In response, the MPAA eventually agreed in 1990 to a new NC-17 rating that would be trademarked, and could only be applied by the MPAA itself.