A slate is a board with a hinge and sticks on either end. The device is designed to keep track and synchronize the different scenes and takes in post-production. The slate has a hinged clapper that indicates to the editor the scene, sequence, takes, or other designation of that particular shot. It also contained blank cards on which both shot details and relevant production notes could be written.
When a movie's sound and picture are out of synchronization, this is known as lip flap. Clapperboards have been essential to filmmaking since the earliest sound films because (until the advent of digital cinematography) visual and audio tracks were recorded on separate media by separate equipment.
The clapper (two sticks hinged together) was invented by F. W. Thring (father of actor Frank Thring), who was head of Efftee Studios in Melbourne, Australia. The clapboard with both the sticks and slate together was refined by Leon M. Leon (1903–1998) a pioneer sound engineer.
The release of the Aretha Franklin 1972 concert film Amazing Grace was delayed for 46 years due to young Academy Award-nominated director Sydney Pollack forgetting to use clapperboards, making the film impossible to edit until modern digital methods were invented.
The clapperboard combines a 'chalkboard slate' with a 'clapstick'. The slate displays the name of the production, the scene and "take" about to be performed, and similar information; an assistant holds the clapperboard so the slate is in view of the cameras, speaks out information for the benefit of the audio recording, then opens the clapstick and claps it shut.
The shutting of the clapstick is easily identified on the visual track, and the sharp "clap" noise is easily identified on the separate audio track. The two tracks can later be precisely synchronized by matching the sound and movement. Since each take is identified on both the visual and audio tracks, segments of the film are easily matched with segments of audio.