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.
Three ways 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.