Friend and inSync reader Chris L. adds to our WFTD on Double System Sound (see WFTD archive Double System Sound):
After recording the tapes are sent to sound transfer where, using the method you describe in Double System Sound, the sound is transferred to Mag (see WFTD above, Mag). The picture is developed and, using the “clapper” at the beginning of a take, an assistant syncs up the picture to the mag by finding the exact point on both picture and mag where the “clap” occurs. Then the editing machine is locked up and the take will run in sync.
All film sound is synced and run this way throughout the entire post production process. The sound track is only printed on the actual film stock for release to theatres so that they can show it to an audience in single system format.