Shooting and Editing on Film
DOI link for Shooting and Editing on Film
Shooting and Editing on Film book
Because of the way film is shot this is what has to happen. At the start of each take the camera assistant shows the camera the clapperboard which in its lower part displays the current shot number and take number. The assistant calls out the number and - if it is a sync shot - claps the upper part of the board down on the lower part. This gives the editor a clear visual and audio point at which to sync up the rushes. If for any reason there isn't time to get a board on the front of a shot it can be end-boarded. The convention then is to offer the board to the camera upside down
If you get to shoot on film you will probably find that clapperboards have moved on a bit since the basic one shown here and now have all kinds of clever digital displays for the numbers and electronic beeps for the sync point. But the principle is the same.