"Start cross" and footage

Picture process	.	Sound process
The sound editor (or the editor himself) resynchronizes picture and sound by aligning the picture clapper-board closure with the sound clapper-board closure (both marked with an X, remember ?) on the film editor. If there is more picture than there is sound, or more generally the other way around, he'll use leader tape to fill the gaps on the other reel.		.
To make sure he won't lose the patiently recovered sync, the editor (or his assistant) will perform the two following operations :
The "start cross" and "punched-out hole"(sorry... I don't know the name for these in english !) After a few feet of leader tape, necessary for threading on the take-up reel, the editor marks an X in black ink named "I told you : I don't know !!!" on the leader tape of each reel. 240 frames further, no more no less (10 seconds at 24 fps), he punches a hole in the picture leader tape. This hole will project a white circle when passing in front of the picture playback system. Aligned with it, on each sound reel, he'll stick one frame of 1 kHz he'll cut in the numerous seconds of 1000 Hz that have been transfered from the location sound recordings to the sprocket tape. During a projection, the white circle must be accompanied by a single "beep", proof that all the sounds are correctly in sync with the picture. 48 frames further, he'll edit the first picture (or the first sound, if the sound starts before the picture) of the scene(s) he's working on.
It is with this sole system of "Start cross and punched-out hole" that sync can be maintained during all the latter processes of post production !!!
The footage To be sure, once again, that he won't lose sync once he starts his editing work (in other words : gets rid of the clapper-boards !), the editor performs a footage of each reel. It consists in jotting numbers with increasing values on the side of the picture or sound reels, in white crayon. These numbers increment every 16 frames in 35 mm and every 40 frames in 16mm. The number starts at 000 on the clapper-board closure of each shot, that number being preceded by the shot n°. ex : 015 006 means we're 6 feet away from the clapper-board of shot 15. This operation can be done by hand, luckily, there are numbering machines that can simultaneously stamp the numbers necessary for the picture and sound reels. The operator must just reset the numbers at each clapper-board closure. If there is any doubt concerning the sync of a series of shots, you just have to inspect the footage numbers of each reel. Each shot, even after it has been edited, will certainly contain several numbers spread along its duration. If they are aligned on all the reel, then there is no sync problem.

Editing (at last !)

The editor can finally start his artistic part of the work ! He'll manipulate the different reels, being careful, with the help of the footage numbers, not to lose sync between the elements.	.	The sound editor can finally tackle the rest of the soundtrack, adding things to the sync sound (usually managed by the editor at the same time as the picture), such as sound effects, atmos tracks and music. He'll start each new reel with a start cross and frame of 1000 Hz, as described previously.
Once his editing is complete, the work copy on which he has performed his edits is sent back to the lab for the negative conformation. There, they will reproduce the same edits on the negative by inspecting where they were performed on the work copy. For dissolves and other effects, conventional symbols (ramps, for instance) are drawn in white crayon by the editor.		The different sound reels :Sync sound, re-recorded dialogue (ADR), foley, atmos tracks, sound effects and music must now be mixed.
This is where the keycodes come into play. They have no relation to the footage numbers, as you'll see below. These numbers are printed on the negative the day it is manufactured. Kodak and Fuji, the only film stock manufacturers, manage so that these numbers are unique to each reel they sell. When the negative is developped, these numbers appear on the positive printed work copy. The lab just has to inspect where the cuts were done, in regard to these numbers, and conform these cuts on the original negative. A long time ago, this negative conformation was performed manually. Nowadays, it is automated, thanks to the bar-code version of the keycode numbers alongside the man readable numbers.
Then several processing stages follow, meant to "equalize" the different shots of each scene seamlessly, by subtly altering the colors, the brightness, the contrast, etc... Sooner or later, the final copy is made, from which the copies the audience will see in the theater are mass duplicated. the picture of the film is ready at last !