A device which is used to convey information about the “take” to the editor (written on the face) while providing a clear synchronization point between audio and picture (using the hinged top portion or “clapper”). 

When the clapper hits the bottom portion it makes a clear snapping sound. This impulse is synchronized to the first frame which contains the image of the two parts touching. 

There are electronic versions of this device which provide timecode information as well.