Pupillary Distance (PD) is the measurement of the distance between the centers of your pupils. This number is important when ordering prescription eyeglasses because it helps to choose the eyeglasses of best comfort. If your pupils distance does not match with your eyeglass where the centers of your pupils are, your vision can be affected– Similar to wearing someone else's eyeglasses. The wrong PD can produce eye strain, fatigue, headaches and blurry vision. If you have a high prescription and the wrong PD these symptoms are often much worse. So, get you exact PD before buying eyewear online.