English: Mechanism of a "split-flap" alarm clock, removed from its case. It displays the time digitally on flaps mounted on wheels. The inset shows the metal tab holding back the top flap. Each complete turn of a wheel advances the wheel on its left by one flap. The wheels are turned by an electric motor (visible in the back) synchronized to the oscillations of the AC current from the wall plug. Therefore, unlike later quartz digital clocks, this is a synchronous clock which is kept to the correct time by the 50/60 Hz AC power grid. The narrow numbered wheel and knobs on the left are the alarm mechanism.