The term B-roll originates from the method of 16 mm film production from an original camera negative. Frames of workprint and of original negative are matched exactly through the use of edge numbers that appeared on each frame of original and work print. But the original was not strung together in a simple linear fashion as was the work print. Instead, the original was edited in a "checkerboard" pattern, with each shot synchronized to an equal length of opaque leader on a second roll. These "A and B" rolls functioned equally to make blind splices, fades, and dissolves possible. Each roll was printed separately onto a single roll of raw stock to produce projection prints. The process is described in the 1982 edition of the "Recommended Procedures" of the Association of Cinema and Video Laboratories, and in the classic text, Film and its techniques.
Then the term B-roll was adopted for the older form of linear-based editing and the common naming conventions used by most television production facilities. Traditionally, the tape decks in an edit suite were labelled by letter, with the 'A' deck being the one containing the main tape upon which the interview material was shot. The 'B' deck was used to run tapes that held additional footage that often supported comments or descriptions made by the interview subject. As linear editing systems were unable to dissolve between clips on the same tape, an edit decision list (EDL) was used to mark clips as "a-roll" and "b-roll" to indicate source machines.
The term "B-roll" is now limited to secondary footage that adds meaning to a sequence or disguises the elimination of unwanted content. This technique of using the cutaway is common to hide zooms in documentary films: the visuals may cut away to B-roll footage of what the person is talking about while the A camera zooms in, then cut back after the zoom is complete. The cutaway to B-roll footage can also be used to hide verbal or physical tics that the editor and/or director finds distracting: with the audio separate from the video, the filmmakers are free to excise "uh"s, sniffs, coughs, and so forth. Similarly, a contextually irrelevant part of a sentence or anecdote can be removed to construct a more effective, succinct delivery. This can also be used to change the meaning of the speaker to fit the view of the producer. In fiction film, the technique can be used to indicate simultaneous action or flashbacks, usually increasing tension or revealing information.
"B-roll" also refers to footage provided free of charge to broadcast news organizations as a means of gaining free publicity. For example, an automobile maker might shoot a video of its assembly line, hoping that segments will be used in stories about the new model year. B-roll sometimes makes its way into stock footage libraries.