Frames.

Film Like David Bordwell, I've been watching a lot of 1940s films lately and wondered often why the title sequences often appear in a little box, frames with back on all sides. In another of his ongoing series of investigations into composition, he offers this explanation:

"Later filmmakers mostly stayed away from corners and edges. You couldn’t be sure that things put there would register on different image platforms. When films were destined chiefly for theatres, you couldn’t be absolutely sure that local screens would be masked correctly. Many projectors had a hot spot as well, rendering off-center items less bright. And any film transferred to 16mm (a strong market from the 1920s on) might be cropped somewhat. Accordingly, one trend in 1920s and 1930s cinematography was to darken the sides and edges a bit, acknowledging that the brighter central zone was more worth concentrating on."

This was doubled down on later when tv screens would cut off the edges of the frame and so credits had to appear in a box so they could be legible in all conditions.