Why do they clap the clapboard before filming a movie scene?

Dear Cecil:

What is the name and purpose of the scissors-top contraption which is clicked shut just before a movie scene is filmed?

Cecil replies:

It’s called a clapboard, and it enables the filmed action to be synchronized at some later date with the recorded sound–e.g., you jiggle the tape around until the sound of the clapboard clapping lines up with the picture thereof.

Send questions to Cecil via cecil@straightdope.com.

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