Cameras, printing machines, projectors, editing machines and some processing machines and almost all other film handling equipment transport film in a manner that prevents the film [and therefore the image] from slipping by using by using standard and precise perforations, often called sprocket holes, to engage with sprockets or teeth, on rollers. Similarly, where an image is intermittently exposed to light [in a camera, printer or projector] the perforations are used to pull the film down a fixed distance, the frame distance, a fixed number of times per second.
In order to permit the smooth transport of film by means of sprockets and claw mechanisms (3D illustrations) in motion picture cameras, printers and projectors, film must be accurately perforated according to agreed standards. The shape and spacing ('pitch') of the perforations depends upon the film gauge and type. There have been many different perforation types over the years for 35 mm and only recently have the different national standards come together. 16 mm has always had more or less the same perforation standards.
The only difference is that in the 16 mm film the soundtrack occupies the space where there would otherwise be a row of perforations. Therefore, 16 mm positive film and all films on which it might be necessary to print the soundtrack have only a single row of perforations. Camera negative 16mm film usually has perforations down each side to maximise a steady, even pull down. 35 mm film has different perforations depending on the purpose for which the film stock is intended. In general, if all film was exposed or projected in an intermittent manner using an image formed by lens system then all film could have the same perforation shape and size. However, most prints are, and have been for 80 years, made by direct contact printing from a negative and in this case, the negative and the print stock are brought into contact and one pull down mechanism or a single set of sprocket teeth transport the "sandwich". To avoid slippage and a blurring of the image the perforations on the print stock are slightly different from the camera stock. It is therefore possible to recognise the different films used in the filming phase and in the laboratory operations by theses perforations.
In the first twenty years of the history of film, there weren't any accepted universal standards for the shape and dimensions of the perforations. Today there are now international as well as national standards and while most are the same, there are still one or two anomalies.
70mm , 65mm, 35mm Film Perforation Types
Many other perforation designs existed in the early years of the century including several systems with round holes. Almost all had four perforations to each frame of the image, as does modern 35mm.