Answer: Audio/Video Synchronization

Even for those of us who have never set foot on a movie set, the scene is a familiar one: the director yells “Action!” and an assistant slams the clapperboard closed with a crack. Although clapperboards have information on them about the director, date, time, and scene, the information is secondary to their primary function as a synchronization tool for the audio and video tracks. The sharp crack sound visual indicator of the two striped sides of the clapper slapping together make it easy to align the two tracks in post production.

The clapperboard itself has undergone several changes over the last century. Originally one assistant would hold the clapper component, called a clap stick, and another assistant would hold the slate with the scene information and date. Leon M. Leon, one of the pioneers of sound engineering, was the first to see the utility of combining the two devices into one unit which allowed a single assistant to hold both the slate and the clap stick. Later revisions included swapping out the slate for a sheet of white acrylic (black marker on a white background is visible under a wider variety of conditions than white chalk on black slate), adding back lighting, and in the most recent incarnations upgrading to digital timers, color calibration bars, and other modern tweaks.