B-roll

The name comes from film and news production, where the primary interview footage was the A roll on one reel and the supporting shots lived on a second, physically separate B roll. The workflow is gone but the label stuck, and it now covers anything layered over the main shot: stock clips, screen recordings, graphics, photos, and generated visuals.

In short form its main jobs are pacing and repair. A cut to b-roll hides a jump cut that would otherwise be jarring, and inserting a relevant shot at the moment a concept is introduced keeps a talking segment from flattening. It also lets one recording carry a lot of visual variety without a second camera or a second take.

The nuance is relevance. Generic stock inserted purely to fill time reads as padding and can pull attention away from the audio, and b-roll that lingers past its point becomes the main shot by accident. Two to four seconds per insert is a common working length, kept under the voice rather than over it so the audio stays continuous.

In a vertical workflow, b-roll usually needs cropping to fill the frame before it can be layered, since most stock and archive footage is horizontal and a letterboxed insert breaks the rhythm of the clip. Matching the colour and brightness of the main shot also keeps each cut from reading as a jolt.