The Academy ratio of 1.37:1 is the standard aspect ratio of a frame of 35mm film when used with 4-perf pulldown, and was standardized by the Academy of Motion Picture Arts and Sciences as the standard film aspect ratio in 1932, although it was used as early as 1928.
Silent films were shot at a 1.33 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 films were introduced in the late 1920s, the soundtrack was placed down a row along the inside of one of the lines of perforations, cutting 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. Therefore, the Academy of Motion Picture Arts and Sciences resolved the problem by trying to retain a similar aspect ratio to the original silent ratio. As the width of the usable image area had been reduced, the height of each frame also was reduced, which created slightly thicker frame lines, which also had the additional benefit of making splices less noticeable in editing. The Academy ratio is actually 1.375, but is usually shortened to 1.37, and often is cited as 1.33 by laypeople, despite the fact that only silent films were shot in that ratio.
Virtually all films from 1932 to 1952 were shot in Academy ratio. However, following the widescreen "revolution" of the 1950s, it quickly became an obsolete production format. Within several years, all major studios started matting their non-anamorphic films in the projector to wider ratios such as 1.66, 1.75, and 1.85, the latter of which is still in common use along with anamorphic (2.39). 1.37:1 is not totally obsolete, nonetheless, and can still be found in selected recent films such as Gus Van Sant's Elephant.
The Academy ratio is not created in the camera, which has continued to use the full frame silent aperture gate for all 4-perf spherical filming. Rather, it is created in the married print, when the optical soundtrack and frame lines are added. Though most non-anamorphic film prints with a soundtrack are now framed to one of the non-anamorphic widescreen ratios, from 1.66 to 1.85, some still retain Academy-sized frames. These frames are then cropped in the projector by means of aperture masks used in the projector's gate in conjunction with a wider lens than would be used for projecting Academy ratio films.
During filming, using the 4-perf frame for widescreen framing when spherical lenses are used is sometimes considered to be wasteful in terms of the cost of film stock and processing, especially in the case of television, which does not require a film print. The 3-perf pulldown process was originally developed by Rune Erikson in 1987 to solve this problem.