This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
When it became clear that nearly all future prints would contain
sound- tracks, the 35mm frame was cropped at the top and bottom
slightly in order to achieve a 1.37:1 frame, nearly matching the old
silent frame. This Academy frame is the basis for nearly all future
`flat' (non-anamorphic) formats, as well as the various television
formats currently in use.
 
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