I was on Wikipedia and found this information on the old movie rating system: 

“Original ratings

The original movie ratings (in use 1968–1970) consisted of:

  • Rated G: General Audiences. All ages admitted.
  • Rated M: Suggested for Mature Audiences. Parental discretion advised.
  • Rated R: Restricted. Persons 16 and under are not admitted unless accompanied by parent or adult guardian.
  • Rated X: Persons 17 and under not admitted.

Originally, the rating system was to have three classification levels ending with Restricted (similar to the rating system used in most Canadian Provinces at the time) however, pressure from theatre owners influenced the MPAA to create a film rating (X) exclusively for adults to protect theatre owners from complaints and legal procedures. Initially, the X rating was not trademarked: under the plan, anyone not submitting his or her film for rating could self-apply the X .”

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