In film and television production, B-roll or B roll is supplemental or alternative footage intercut with the main shot.
16 mm film splices
The term B-roll originates from a particular solution to the problem of visible splices in the narrow film stock used in 16 mm film. 35 mm film was wide enough to hide splices, but 16 mm film revealed the splices as flaws in the picture. To avoid this problem, the intended shots were spliced to opaque black leader, with the black leader hiding the splice. Two sequences of shots were assembled, the odd-numbered shots on the A-roll, and the even-numbered shots on the B-roll, such that all of the shots on one roll were matched by black leader on the other roll, in a checkerboard pattern (an alternate name for the process was "checkerboard printing".) Unexposed 16 mm raw print stock was exposed twice, once to the A-roll, then it was exposed again to the B-roll.
Linear editing era
In the 1980s, the term B-roll was adopted for linear video editing using at least two video tape machines. Traditionally, the tape decks in an edit suite were labeled by letter, with the 'A' deck being the one containing the main tape upon which the main action material was shot. The 'B' deck was used to run tapes that held additional footage such as establishing shots, cutaway shots, and any other supporting footage. The sound was usually taken from the A deck alone, so that the B deck provided video without sound. 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.
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