|Film Sprocket Holes. Nikon D7000|
Not all holes are round; these are sort of rectangular with a bit of a curve on them. They are rather useful though for pulling the film through in the camera.
In my little pinhole camera, I have a small piece of plastic inside that makes a 'click' as I haul the film through just using brute force and tension.
I am not sure if the click would sound the same if the holes were round.