The Motion Picture Association of America film rating system was intiated on November 1, 1968. The only rating that was not trademarked by the MPAA was its X rating for film that contained content that was not suitable for children, such as nudity or extreme violence, with the idea being that producers could sidestep applying to the MPAA if they wished to self-apply the X rating to their film. Midnight Cowboy won Best Picture in 1969 with an X-rating, although it was reclassified as R in subsequent releases.
X-rated feature to be nominated for Best Picture
was Kubrick's A Clockwork Orange (1971). It received
a total of four Oscar nominations: for Best Picture, Best Director,
Best Adapted Screenplay and Best Film Editing (losing all four awards
to The French Connection). The year after its release, Kubrick
30 seconds of footage to earn an R rating.
only other X-rated film to receive any nominations was Last Tango in Paris in 1973, which was nominated for
Best Actor (Marlon Brando) and Best Director (Bernardo Bertolucci).
By that point the X rating had acquired a stigma since because of its non-trademarked status, it had been taken over by producers of pornography. It ultimately came to pass that an X rating and pornography became synonymous even though that was never the original intention of the X rating. Even after the MPAA replaced the X rating with NC-17 in 1990, many major theatre chains or media outlets refuse to carry or accept advertising for those films despite many of them - Henry & June (1990), Tie Me Up! Tie Me Down! (1990) and Shame (2011) - being critically lauded.
the other end of the spectrum, no film has won the Best Picture
Oscar with a G-rating since Oliver! in 1968.