3-perf

Negative pulldown is the manner in which an image is exposed on a , described in the number of film perforations spanned by an individual frame. It can also describe the orientation of the image on the negative, whether it is captured horizontally or vertically. Changing the number of exposed perforations allows a cinematographer to change both the aspect ratio of the image and the size of the area on the film stock that the image occupies (which affects image clarity).

The most common film pulldowns for 35 mm film are 4-perf and 3-perf, the latter of which is usually used in conjunction with Super 35. 2-perf, used in Techniscope in the 1960s, is enjoying a slight resurgence due to the birth of digital intermediate techniques eliminating the need for optical lab work. Vertical pulldown is overwhelmingly the dominant axis of motion in cinematography, although horizontal pulldown is used in IMAX, VistaVision (still in use for some visual effects work), and in 35 mm consumer and professional still cameras.

Most 35mm film systems, whether they are cameras, telecine equipment, optical printers or projectors, are configured to accommodate the 4-perf system, where each frame of 35mm is 4 perforations long. 4-perf was (and remains) the traditional system, and virtually all projectors are based on 4-perf, because 4 perforations is the amount needed per frame vertically in order to have enough negative space for a roughly squarish image, which became the silent film standard aspect ratio of 1.33:1.

Later, when the film industry was facing the perceived threat of obsolescence to television, which also was universally a 1.33:1 aspect ratio at the time (and remained so in many countries until the introduction of DTV), studios started experimenting with various competing widescreen formats.

...
Wikipedia