The pupillary distance (P.D.) is the measurement between your pupils, or the centers of your two eyes. It varies slightly depending on if you're looking to distance or near. This measurement ensures the accuracy of lens placement in your chosen frame. This can be the difference between seeing well with your new glasses or having mild to major problems. This is not considered a part of your glasses prescription or eye exam. We assume responsibility for the quality and accuracy of your glasses when purchased through us, therefore we do not measure this if you are purchasing your glasses elsewhere.