Your PD is in fact your PUPILLARY DISTANCE. This is the measured distance in millimeters from one pupil to the other pupil. When taking measurements for eyewear (especially progressive and multi focal lenses) it is critical that this distance be exact and accurate. The left and right pupil are often not equidistant from the bridge of the nose. The ideal way to measure PD is to do this monocularly from the center of the bridge to each pupil individually, for a more accurate and true measurement.