Clapperboard

A clapperboard is a device used in the movie industry to synchronize sound and video. A typical clapperboard consists of a board or slate with a hinged piece of wood attached. Details of the shot being filmed are written on the board which is held in front of the camera at the beginning (or sometimes the end) of that shot. These details are also read aloud and then the clapper is literally clapped against the board. The frame at which the clapper closes can then be matched to the corresponding clap on the soundtrack, allowing the sound and picture to be married together for editing purposes.

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Clapperboard was also the name of a 1970s BBC children's television quiz show in which child contestants had to answer questions based on movie clips.

I think the presenter may have been John Craven but I'm not certain