Motion Picture Association of America Voluntary Movie Rating System 1968

G—General Audiences: All ages admitted

PG—Parental Guidance suggested: Some material may not be suitable for children. The PG moniker was originally “M” for mature audiences, but many parents regarded the “M” a “sterner rating” than the R category. The mature label was quickly changed to GP, standing for “General audiences, Parental guidance suggested). In 1970 the GP label was revised to the current “PG” for Parental guidance suggested.

PG-13—Parents Strongly Cautioned: Some material may be inappropriate for children under 13. The PG-13 label was added on July 1, 1984, to address complaints from parents that many PG movies had a “higher level of intensity” than others.

R— Restricted: Under 16 requires accompanying parent or adult guardian. The age limit was soon changed to “Under 17” and in some jurisdictions is as high as “Under 18.”

X—No One 17 and Under Admitted. This label was originally “No one 16 and under” but was quickly raised to 17. The X label was changed to  NC-17—No One 17 and Under Admitted, on September 27, 1990, to address the negative connotations many had about the X rating.

Source: Valenti, J. (1969). Special statement to readers of social education. Social Education, 33, 146–151; see also http://www.mpaa.com.