There are two types of pupil distance (PD). There's a distance PD which is what we need and a near PD.
Pupil distance is always written in millimeters (mm) and the larger number is always your distance PD.
Now, it can be written two ways. Monocular or Binocular. When a PD is taken monocularly you will see a reading of about 25-35mm for each eye. If taken binocularly you will see something like 55-75mm total PD.
This measurement is important.
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.
How to get your P.D.
+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.