Part of the MPAA
's original rating
s system for US
films, a liberalization and modernization of the old Hays Code
. There was G
, for general audiences (it now means "kiddie glop"). And M
, "for mature
audiences"; confusion over what "mature" meant led to its being renamed GP
, equivalent to today's PG
("parental guidance suggested", which has forked into PG and PG-13). R
was always R - no one under 17
gets in sans adult
X meant something different from today (now it would be NC-17); "art films", with nudity or "bad" language got an X. Midnight Cowboy, a celebrated picture for its day, was X-rated. In the 70s, the porn market seized upon it, hyping it up to "XXX"; Hollywood backed away from the letter. So, for many years, studios fought with the MPAA over a film here, a film there, that had too much stuff in it to get the R rating. NC-17 became the eventual compromise, allowing Miramax to release Henry and June without hacking it to R-rated pieces.
Some protesters now try to raise the same sort of fuss over NC-17 content that many did over X in the old days, and I'll bet some retailers don't sell or rent NC-17 videos.