Aperture plates are used to mask the image in the gate. Projectors use either single aperture plates or multi-format "sliding" aperture plates (commonly found in turret projectors). Plates are cut for each aspect ratio a booth might show, and are cut for each projector and screen, using test films to get each aspect ratio as close to SMPTE specifications as the lenses and masking will allow.
Each projector manufacturer (Century, Simplex, Kinoton) has different aperture plates, and most vary model to model. Most are not interchangeable, with the exception of Ballantyne Pro 35 and most Kinoton/Norelco/Phillips models.
"Fuzzy Edges" and Masking: Even the most perfectly cut aperture plate will not have a perfectly sharp edge. Because the film and the edge of the plate are not on the same focal plane, there will be a slight shadow on all edges of the image. To eliminate the soft edge, most venues will bring their masking in to cover these shadows. It is common practice to over-cut aperture plates very slightly so that the shadow spills onto the masking.
Cutting Aperture Plates: Aperture plates are typically cut using flat files that are blank on two sides. Ideally the technician or projectionist will begin with a plate that is "pre-cut" for their aspect ratio, but in some cases will have to use what is available, for example cutting a 1.66 plate out of a 1.85 plate. IMPORTANT: Projectors must be cleaned thoroughly after plates are cut. Stray bits metal can work their way into the film path and scratch the film.