VPH (Vibrations Per Hour) refers to the oscillation of the balance of a mechanical watch. The most common frequency for modern mechanical wristwatch movements is 4 Hz (or 28,800 vph). A watch beating/ticking at 4Hz makes 4 oscillations per second (or 8 semi-oscillations) vibrations per second. This means that 8 x 3600 seconds in an hour = 28,800 vibrations per hour.