Unless you're surfing Skinemax, "X-rated" doesn't mean much anymore. But, for a few decades, that decisive letter symbolized Hollywood's dreaded stamp of rejection and certain commercial doom. The MPAA (Motion Picture Association of America) formed in the 1920s to become the regulator and censor of the film industry. In 1968, a four-tier ratings system — G, M, R, and X — went into effect, with Brian De Palma's Greetings landing the first-ever X rating for its quasi-shocking sexual content that year.
From there, a surprising number of future critical and cult classics were branded with the dramatic rating — most going through a series of edits to be reassessed something more palatable, though some embraced their outlaw status. This censorship dance went on until 1990, when the MPAA threw out the X rating in favor of the less drastic NC-17, which simply means no one under 17 will be admitted at a theater. (Even that rating is rare in a final product; for context, Scream was originally NC-17 before a careful edit).