In 1946, a request was made to replace the British Board of Film Censors with an independent censorship board; this triggered the formation of a committee headed by Professor K.C. Wheare to examine the film censorship processes in the United Kingdom. The results, in 1950, was the Report of the Departmental Committee on Children and the Cinema 1948, usually called the Wheare report, which advise the creation of a new rating system:

X - films aimed at adults only, from which children should be absolutely excluded;

C - films aimed specifically at children;

Advisory U - films suitable for all;

Advisory A - films suitable for all, but dealing with more adult subject matter.

In 1951 the British Board of Film Censors did not accept all of the committee's suggestions, but it did issue a new certificate, the "X certificate," to replace the "H certificate" which had until then mostly applied to violent horror films. The wording on the "X certificate" was:

This film has been passed for exhibition to persons over the age of sixteen.

The "X-rating" was adopted by the Motion Pictures Association of America (MPAA) in 1968 - this was very obviously inspired by/based on the British rating system ("Persons Under 16 Not Admitted").

The British raised the age limit on their X-certificate to 18 in 1970, and it was abolished in 1982. The MPAA raised the age limit for X-rated films to 17 in 1970, and in 1990 retired the X-rating, replacing it with the NC-17 rating. In Canada, film classification is done at the Provincial level, and Ontario still has an RX (Restricted-Adult Sex) rating.

One of the immediate effects of the new rating was a taboo association with the letter X; this was used by Hammer Horror studios in films like The Quartermass Xperiment (1955). To quote The Routledge Companion to British Cinema History:

The "X" rating became another exploitable commodity in the independent distributor or exhibitor's arsenal, its prominent position on the marketing material serving to lure audiences just as effectively as the imagery or title of the film itself. Jacey cinemas would thrive in this feverish atmosphere, where any "X"-rated film seemed guaranteed to bring punters in off the streets.

It should be noted that initially the "X" rating had nothing to do with pornography, and applied to any film which because of content (sexual or otherwise) was deemed unsuitable for children; "stag films" and the like weren't distributed through normal channels and didn't have a rating anyway.

The "X" rating was not universal by any means - various film rating classification systems have used different letters/symbols for adults-only films. The use of XXX and the like by the adult film industry, however, goes back to an old beer rating system - "X" was a traditional European grade for a strong beer, and stronger beers were marked "XX" (double x) or "XXX" (treble x or triple x); which remains today in some of the marketing materials for certain brands. Americans appropriated and extrapolated on the reference - there are cartoons where moonshine jugs are labelled "XXX" or more, for example this frame from Disney's "When The Cat's Away" (1929). In just the same way, pornographers played off the "X" rating of films to suggest that their material was even stronger and more taboo, resulting in the ubiquitous "XXX" in the adult entertainment industry.

[/edit] Fixing spelling.