Double-system recording is the standard procedure on motion pictures that are originally photographed on film.
This procedure requires that both camera and sound recorder share a very accurate time reference, and that the speed of the camera and sound recorders be carefully governed.
As quartz-based timers came into common use, film cameras and sound recorders adopted these, and these were accurate enough to remove the need for an interlock cable.
Double-system recording requires that sound and picture be manually synchronized at the start of every "take" or camera run.
A clap sound on the recording is matched to the closed clapper image on the printed film, and thus the two recordings can be realigned into sync.