What's the use of using a black and white board before making a movie?

That board is called "scoreboard", which has information such as the number of times, the number of scenes, the director, the title of the film, the film company and so on.

When making a movie, the picture and sound are recorded on different media (the previous picture is film and the sound is tape). In the later stage of editing, it is necessary to synchronize sound and picture. If there is no reference, the work will be very difficult. When the slate is closed, the sound is very crisp, and the clear and obvious waveform can be seen on the audio track. At the same time, it is not difficult to find the border when the slate has just closed the screen. Through the matching of the two, the perfect synchronization of sound and picture can be realized.

In addition, when making a movie, it is not taken in the order of the script, and the material is often chaotic. When playing the board, the recorder should also tell the corresponding information of the next time, and the photographer and the recorder should record the information of the next time at the same time, which is also for the later synchronization of sound and picture.