Filming & Production
Slate
Also called: clapperboard, clapper, sync clap
Two jobs sit in one object. The written information identifies which take is which when you face 90 clips and no memory of the day, and the clap creates a single frame where the sticks meet with a matching spike on the audio waveform, which is how separately recorded sound gets lined up with picture.
Software that syncs by waveform analysis has made the clap less necessary, though it struggles on noisy locations and long takes, and clapping costs nothing. Solo creators get most of the benefit from a hand clap in frame and saying the take number out loud.