In ENG (electronic news gathering) work, b-roll refers to the extra scrap footage that you shoot that gives the editor something to cut to when the main footage is unusable.

For example, say the mayor is opening a new building. Watching his talking head speak for 45 seconds would be boring. Plus, if you edit a longer speech down to 45 seconds, there would be a great many jump cuts in the footage. So, you fill up your b-roll with pictures of the exterior of the building, the crowd looking interested, etc. Then, later when you are editing, whenever you want to cover a jump cut or get bored of the mayor's face, you can switch the video to b-roll footage, while leaving the audio intact.

Effective use of a b-roll can also make it look like you shot an interview with multiple cameras, when in fact, you only had one.

This style of editing is why editing from two players to a single recorder is known as editing a b-roll.

The "B-roll" got its name from early TV news programs, though it technically came from the film world. Back in the day, news stories were shot on film and played back live on the broadcast. The editors would assemble an "A" roll or reel which had the primary sound, narration, and interviews. They would also assemble a "B" roll/reel that had all the visual elements and cutaways and visual aids that were relevant to the story. As the reel played live on the broadcast, a technical director would switch between the "A" and "B" reels in order to tell the story on the news.

Today, the news footage is edited together, but the name stuck. Any footage that's used to visually illustrate a story is now known as a "B-roll." This includes, slides, on-screen quotations, and archive footage. It also comes in handy as a cutaway shot if you need to cover an edit, or to hide a mistake an onscreen character makes by switching to another scene.

Source:
http://www.texasb-roll.com/ (They sell B-roll footage of Texas)

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