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In some countries, X or XXX is or has been a motion picture rating reserved for the most explicit films. Films rated X are intended only for viewing by adults, usually legally defined as people over the age of 18 or 21.
The Australian Classification Board (ACB), a government institution, issues ratings for all movies and television shows sold or aired. Material showing explicit, non-simulated sex that is pornographic in nature is rated X18+. People under 18 may not buy, rent, exhibit or view these films. The exhibition or sale of these films to people under the age of 18 years is a criminal offence carrying a maximum fine of $5,500. Films classified as X18+ are banned from being sold or rented in most Australian states. They are legally available to be sold in the Australian Capital Territory and the Northern Territory. Importing X18+ material from these territories to any of the Australian states is legal, as the constitution forbids any restrictions on trade between the states and territories. The X18+ rating does not exist for video or computer games.
Films may be shown in theaters in France only after classification by an administrative commission of the ministry of Culture. In 1975, the X classification (officially: "pornographic or violence-inciting movies") was created for pornographic movies, or movies with successions of scenes of graphic violence. The commission has some leeway in classification, it may for instance take into account the artistic qualities of a movie not to count it pornographic. Movies with an X rating may only be shown in specific theaters (which hardly exist nowadays in France); they bear special taxes and tax rates, including a 33% tax on revenue. In 2000, some conservative associations sued the government for granting the movie Baise-moi, which contained graphic, realistic scenes of sex and violence, a non-X classification. The Conseil d'État at litigation ruled that the movie should have been rated X. The decision was highly controversial, and some suggested changing the law under which it was rated 18.
United Kingdom 
The original X certificate, replacing the H certificate, was issued between 1951 and 1982 by the British Board of Film Censors in the United Kingdom. It was introduced as a result of the Wheare Report on film censorship. From 1951 to 1970, it meant "Suitable for those aged 16 and over," and from 1970 to 1982 it was redefined as meaning "Suitable for those aged 18 and over". The X certificate was replaced in November 1982 by the 18 certificate. Sometimes the rating of a film changes significantly over time. For example Plan 9 from Outer Space received an X rating in 1960 that was changed to a PG rating in 1990. In some early cases, films received an X rating due to political content. The Battleship Potemkin was rejected for "inflammatory subtitles and Bolshevik propaganda" in 1926, rated X in 1954, and finally rated PG in 1987.
United States 
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, 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.
See also 
- Henry & June (1990), first film given new NC-17 rating
- Production Code
- Re-edited film
- Strong language
- Television rating system
- .xxx, top-level domain
- Screen Online article about the X certificate
- Refused Classification Website covering in varying detail many films that have run foul of the Australian Office of Film and Literature Classification, with separate sections for hardcore films and computer games
- Explanation of X-ratings in the US
- How “X-rated” Came to Mean “Porn” and the Death of Movies for Grown-ups A brief history of the social and legal forces that drove adult themes out of the legitimate cinema, by film director Tony Comstock