Academy format

The Academy ratio of 1.375:1 (abbreviated as 1.37:1) is an aspect ratio of a frame of 35mm film when used with 4-perf pulldown. It was standardized by the Academy of Motion Picture Arts and Sciences as the standard film aspect ratio in 1932, although similar-sized ratios were used as early as 1928.

Silent films were shot at a 1.33 aspect ratio (also known as a 4:3 aspect ratio), with each frame using all of the negative space between the two rows of film perforations for a length of 4 perfs. The frame line between the silent film frames was very thin. When sound-on-film was introduced in the late 1920s, the soundtrack was recorded in a stripe running just inside of one set of the perforations and cut into the 1.33 image. This made the image area "taller", usually around 1.19, which was slightly disorienting to audiences used to the 1.33 frame and also presented problems for exhibitors with fixed-size screens and stationary projectors.

From studio to studio, the common attempt to reduce the image back to a 1.33:1 ratio by decreasing the projector aperture in-house met with conflicting results. Each movie theater chain, furthermore, had its own designated house ratio. The first standards set for the new sound-on-film motion pictures were accepted in November 1929, when all major US studios agreed to compose for the Society of Motion Picture Engineers' (SMPE) designated size of 0.800 in × 0.600 in (20.3 mm × 15.2 mm) returning to the aspect ratio of 1.33:1.

Following this, Academy of Motion Picture Arts and Sciences (AMPAS) considered further alterations to this 1930 standard. Various dimensions were submitted, and the projector aperture plate opening size of 0.825 in × 0.600 in was agreed upon. The resulting 1.375:1 aspect ratio was then dubbed the "Academy Ratio". On May 9, 1932, the SMPE adopted the same 0.825 in × 0.600 in (21.0 mm × 15.2 mm) projector aperture standard.

...
Wikipedia