First, let us start by asking what the heck is a PD?
Don’t worry you don’t need a PhD to get your PD… So what is PD’s then?
PD is short for Pupillary Distance, the distance measured in millimeters between the centers of the pupils of the eyes
Many places will measure it as one measurement for example 63mm, the problem with that is when the lenses are made, the lens technicians place each lens these same distance away from the centre of the lens (31,5mm). Unfortunately most peoples eyes are not exactly the same distance from the middel of your face/nose, for example 30mm and 33mm (see image below).
You can see above that both are 63mm Pd’s, but in the first case if they just divided the PD in half, the lenses will have been place 1,5mm off centre causing possible prismatic effect, headache or discomfort.