A ‘single’ pupil distance is the distance between the pupils of two eyes. A ‘dual’ pupil distance is the distance from the pupil of each eye to the center of your nose. A single PD will be just one number, while a dual PD will have a number for each eye, marked ‘right’ and ‘left’. (Some prescriptions may have ‘OD’ for the ‘right’ eye, and ‘OS’ for ‘left’ eye.)