The distance between the pupils of your eyes is pupillary distance. It is measured from the center of one pupil to the center of other pupil. This measurement is used while making prescription eyeglasses. The range of adult PD is between 54-74 mm while that of a child falls between 43-54 mm.
Pupillary distance is essential when going for a pair of prescription eyeglasses. Every prescription lens has an optical center that is determined by distance between the pupils. Knowing this distance helps in positioning the lenses correctly in your eyeglasses frames so that you can see properly through the lenses. Hence, the pupillary distance needs to be as accurate as possible because it determines where you look through your eyeglasses lenses.