A clapperboard, also known as a dumb slate, clapboard, film clapper, film slate, bertolo clapz, movie slate, or production slate, is a device used in filmmaking, television production 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. It is operated by the clapper loader. It is said to have been invented by Australian filmmaker F. W. Thring. Due to its ubiquity on film sets, the clapperboard is frequently featured in behind-the-scenes footage and films about filmmaking, and has become an enduring symbol of the film industry as a whole.

In the silent era the principal requirement of film stock identification during a day's shoot was the slate.

The clapper as two sticks hinged together is credited to F. W. Thring (father of actor Frank Thring), who later became head of Efftee Studios in Melbourne, Australia. The invention, however, is thought to be the responsibility of two of his sound engineers: Alan Mill and Harry Whiting. The clapperboard with both the sticks and slate together was a refinement of Leon M. Leon (1903–1998), a pioneer sound engineer.

The clapperboard combines a chalkboard slate or acrylic board with a set of clapper sticks across the top; one stick is fixed to the upper edge of the slate, while the other is attached to it by a hinge at one end. The slate displays the name of the production, the scene and "take" about to be performed, and similar information; a camera assistant holds the clapperboard so the slate is in view of the cameras with the clapper sticks already open, speaks out information for the benefit of the audio recording, then snaps the sticks shut. Chalk or dry-erase markers may be used to write on the slate or whiteboard respectively.

The shutting of the clapper sticks 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 expressly identified on both the visual and audio tracks, segments of film are easily matched with corresponding segments of audio.

Finding a way to synchronize visual and audio tracks was essential to traditional filmmaking because film stock reacts to light, not sound. During a film shoot, the audio track was always recorded by the audio engineer with a separate system on separate media (so-called double-system recording). (For early sound films, playback of the audio track was synchronized during post-production with sound-on-disc techniques; engineers later figured out how to directly add an audio track to a release print with sound-on-film techniques.) Failure to use clapperboards can prevent the film editor from synchronizing the visual images on film footage with the accompanying audio recordings, as actually happened with the long-delayed film Amazing Grace.

Methods were later developed to directly record sound to film as part of a single system integrated with the film camera (so-called single-system recording), which was most commonly used with small formats like Super 8 film. However, single-system recording did not render clapperboards obsolete. First, single-system recording of sound-on-film is "decidedly inferior in audio quality" to traditional double-system recording. Second, footage from single-system recording is difficult to shoot and edit. Since the sound playback head cannot block the projector gate and must be placed after the gate, the soundtrack must be offset by several frames (usually 28, 26, or 18 ahead) to maintain sync with the frame in the gate. With such footage, cutting to the next shot when an actor's lips stop moving will risk cutting off their last syllable, unless the soundtrack is copied and edited on a separate system, and actors must be directed to pause to allow for such cuts. Because of these technical limitations, the film industry has continued to use double-system recording for professional-quality film projects.

The development of videotape made single-system recording less inconvenient, since video and audio signals corresponding to the exact same point in time could now be captured and stored together as magnetic signals on the same medium. Despite that, contemporary digital cinematography still relies upon double-system recording, which means that directors of professional-quality film and television productions still need to use clapperboards. The main reason is that the audio features needed for quality single-system recording are found only on midrange or "prosumer" digital cameras. Low-end cameras omit those features for cost reasons. High-end professional cameras omit those features because manufacturers assume that a professional film crew will follow the well-established tradition of hiring a sound engineer (along with other sound specialists to form a sound department) who will bring along and use dedicated audio recording equipment.