The B-roll carried video — typically visual filler — that wouldn’t work on the A-roll. The director could cut between the two reels. If he was feeling fancy, sometimes he’d even dissolve. All this would happen on the fly — the a-roll, the b-roll and the audio cart. And they’d hit supers too.

A mere fifteen years ago, ENG editors would make A and B rolls when they wanted to do a lot of dissolves or other effects. It was like a big puzzle, with black holes on the A roll coinciding with the corresponding pictures on the B roll and vice versa. You should try it.

Like, next time you shoot a story, whenever that might be.