This article is from the rec.arts.movies.current-films FAQ, by Evelyn C. Leeper email@example.com with numerous contributions by others.
These are cue marks, or "reel-change dots," signaling the projectionist
that it is time to change reels. There is actually a set of dots.
Four consecutive frames are marked with a little circle in the upper
right-hand corner of the frame. The first set (4 frames) of cue marks
(the motor cue) is placed 198 frames before the end of the reel. (198
frames is 8.25 seconds, or 12.375 feet.) There are 172 frames between
the first set of cue marks and the second set of 4 frames, the
changeover cue. There are 18 frames between the changeover cue and the
runout section of the trailer (or foot) leader. The projectionist
threads up the next reel of film so that he has about nine feet of
leader between the lens and the start of the film. At the first cue
mark, he starts the motor on the second projector. This gives the
projector time to get up to to speed and for the speed to stabilize.
On the second cue mark, he throws the switches that change the picture
and sound sources. In some old films on TV, you'll see long changeover
cues since some projectionists were paranoid that they would not see
Video versions usually do not have these dots because when the transfer
was made, the original negative was used, or a postive that was made
from the original negative was used. Sometimes an interneg is used.
In any event, only prints that make it to the theatre have the
change-over dots. For older movies, sometimes the only available
print is a release print, which means the dots will appear.
(Paul Parenteau [dog@sequent.COM], Ron Birnbaum [firstname.lastname@example.org], Harris
Minter [email@example.com], Jeffry L. Johnson
[firstname.lastname@example.org], and Mike Brown