A device consisting of a fixed circle of calibrations over which is a manually turned disc with an aperture. This allows information engraved on the lower scales to be read in conjunction with information engraved on the rotating scale.

The most common example is the lunar volvelle which shows the age and the phase of the moon as it develops over 29 1/2 days.

Vibration per Hour

For a watch movement has 36,000 VPH, the balance wheel swings back-and-forth 18,000 times or 5 Hz.