PD stands for Pupil Distance. The P.D. let's us know where to place the lenses in front of your eyes so to not create any unwanted prism in a lens. Prism will cause eye strain and fatigue. If glasses are made without the patient's correct P.D., it could be all for nothing. Three ways to get your PD: Ask your eye doctor to take the measurement during the exam. Call the last place you bought glasses and ask for it. Ask your local optical professional to take the measurement.