What is a PD?
A PD is one of a number of measurements to determine where to place the optical center of your prescription lenses into your frame. The PD tells us, horizontally, how to position the lens in front of your pupils.
While wearing the frame, we also take an OC (optical center) measurement which tells us the vertical position for the lens.
A PD measurement can be monocular (one per eye) or binocular (distance between eyes). A monocular measurement is most accurate as it captures any asymmetry based on how the frame sits on the bridge of your nose.