CinemaScore is a market research company that polls audiences on opening night and gives movies grades based on those scores.
Here's how they describe themselves:
Since 1978, CinemaScore has been polling moviegoers at major movie releases on opening night to collect demographic information and calculate a distinctive CinemaScore grade.
CinemaScore has only given 21 movies an F rating.
The first movie to get an F rating was Eye Of The Beholder in 1999.

And the last movie to get an F rating was The Turning in early 2020.
