Synchronously linking multiple film reels or audio tracks together — classically with perforated tape and sprockets in analogue cutting. Digitally: timecode sync.
In analog editing, you would sit in front of a Steenbeck or Moviola and have to run multiple film reels in sync – that is coupling. You would link the reels together using perforations tape and sprockets so that the picture and sound ran exactly parallel. If the tracks were out of sync by even one perforation, you would immediately hear the discrepancy: the sound would be ahead or behind. That's why coupling was a fundamental skill for every editor – a physical, manual craft.
The practice was tricky. You would load the first reel (usually picture material) into the machine, thread it over the guide rollers, and tension it. Then you would take the sound track – for 16mm or 35mm, often multiple audio reels for different soundtracks – and attach it to the picture reel with perforation tape clips. The perforation tape would loop through the perforations of both film strips; then you would slide it into the Steenbeck's special locking mechanism. The sprocket would drive all coupled reels evenly. If a clip was crooked or the perforation tape was too loose, the entire editing rhythm could be thrown off.
Multitrack productions (for example, a cinema mix with dialogue, music, and effects on separate reels) required patience and concentration. You had to remember the order, not mix up the reels, and handle all the tapes simultaneously when winding forward and backward – no automation helped you with this. Reels would often collide or the perforation tape would tear, and then re-coupling was necessary.
In digital editing, coupling has become a ghost concept. The editor uses timecode sync or track linking in the NLE (Avid, Premiere, Final Cut): picture and sound are connected within the same clip container or via timecode references. One click, and they follow each other. No perforations, no sprockets, no physical sources of error. However, those who work with old material or understand 16mm editing know the disciplining power of coupling – it enforced precision.