In film and television production, B-roll , B-roll , B-reel or B-reel is supplemental or alternative footage intercut with the hand shot.  The term A-roll referring to the main footage has fallen out of use. 
Film and video production
Movies and videos can be seen from the backdrop of the story . Establishing shots can be used to show the audience the context of the story. These secondary images are often presented without sound, and are expected to continue. The various shots presented without sound are called B-roll . 
B-roll may be shot by smaller second unit crews, since there is no need for sound. In film, smaller MOS cameras can be used for greater portability and ease of setup.  In electronic news-gathering (ENG) and documentary filmprojects, B-roll footage is often shot by the interviewer.  In a docudrama project, B-roll can be read to dramatic re-enactment scenes staged by the producer and performed by actors, to be used as cutaway shots. 
B-roll footage can be added to a stock footage from a stock footage library.  
16 mm movie 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 splice as flaws in the picture. To avoid this problem, the target shots were spliced to opaque black leader , with the black leader hiding the splice. Two sequences of shots are assembled, the odd-numbered shots on the A-roll, and the same-numbered shots on the B-roll, such that all of the shots on one roll a checkerboard pattern (an alternate name for the process was “checkerboard printing”.) Unexposed 16 mm raw print stock was exposedtwice, once to the A-roll, then it was exposed again to the B-roll.   
Until the mid-1970s, ENG teams shot both A-roll and secondary B-roll footage on 16mm film. Sound was integrated onto the film by a magnetic stripe at the edge of the film. The A-roll and B-roll scenes, shot at 24 frames per second, Were converted to the television frame rate of 30 fps using a telecine system consistant en two movie projectors, one showing the hand A-roll footage and the other showing the B-roll. The sound of the A-roll footage was used, or sound from narration or voiceover, while MOS images from the B-roll were intercut as desired. 
Linear video tape editing
In the 1980s, the term B-roll was adopted for linear video editing using at least two video tape machines. Traditionally, the tape in an edit suite was labeled, with the ‘A’ deck being the one containing the main action. The ‘B’ deck has been used to create additional shots, cutaway shots, and any other supporting footage. The sound was usually taken from the deck, so that the 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”
- Camera coverage
- Pick-up (filmmaking)
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