Double-system recording is a form of sound recording used in motion picture production whereby the sound for a scene is recorded on a machine that is separate from the camera or picture-recording apparatus. Double-system recording requires that sound and picture be manually synchronized at the start of every "take" or camera run. This task was performed by the clapper slate. A clap sound on the recording is matched to the closed clapper image on the printed film, and thus the two recordings can be realigned into sync.