This needs expansion. Anyone willing can expand this.
Originally, there were 4 ratings. G was all audiences, all ages. M was all audiences, but parental discretion was advised. R was for people ages 16 and up unless under 16 and accompanied by someone. X was for adult films. Nobody under 18 was permitted for X films.
In 1970, the system changed. M was changed to GP. R was changed from under 16 to under 17 for adult accompaniment. X now lets 17 year olds in.
GP replaces PG, but have the same rules.
In 1984 PG-13 was added, closing the gap between PG & R.
X replaces NC-17, and still uses the rules X did.
First use of NC-17, with 17 and older only allowed.
In 1996, NC-17 was raised back to 18 minimum for age.
New designs with descriptions replaced the 17-year-old previous rating designs.