A clapperboard is a device used in filmmaking and video production to assist in synchronizing of picture and sound, and to designate and mark the various scenes and takes as they are filmed and audio-recorded.
Other names include clapper, clapboard, clacker, slate, slate board, slapperboard, sync slate, time slate, sticks, board, smart slate, dumb slate  and sound marker. 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.
How they work
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.
Traditional clapperboards consisted of a wooden slate with a hinged clapstick attached to its top.
Smart slates or digislates are electronic SMPTE time code versions with digitally displayed information, and in some productions created in the digital domain, electronically superimposed versions have supplanted physical clapperboards.
A verbal identification of the numbers, known either as "voice slate" or "announcement", occurs after sound has reached speed.
A clapper board is generally used to identify all takes on a production, even takes that do not require synchronization, such as MOS takes, which have no sound. When a slate is used to mark an MOS take, the slate is held half open, with a hand blocking the sticks, or closed, with a hand over the sticks.
The clapper loader (or 2nd AC) is generally responsible for the maintenance and operation of the clapperboard, while the script supervisor is responsible for determining which system will be used and what numbers a given take should have. While these are usually fairly obvious once a system has been agreed upon, the script supervisor is usually considered the final arbiter in the event of an unclear situation.
Sometimes a "tail slate" or end slate is filmed at the end of a take, during which the clapperboard is held upside-down.