The PD/Pupillary Distance is the distance between your pupils, it ensures the correct centration of your Prescription lenses in front of your eyes.
On average the PD in an adult is usually between 60 and 66 millimeters. If your prescription shows two PDs for example 64/61 this will mean the 64mm will be used for distance glasses, bifocals or progressive lenses and the 61mm will be used for reading glasses.
If you only have one PD this will probably be your distance PD, you can reduce this by 3mm if you require reading glasses, as your eyes converge to read, however, we would suggest confirming this with your optician to be certain.
Sometimes your optician may write your PD for each eye, for example, 32/33, this is called a mono PD, you just need to add these together to arrive at your binocular PD, you can also add a note in the additional info box when ordering to list your mono PD measurements if you wish, this will be useful if ordering Varifocals/Progressive lenses.
For further information about Pupillary Distances, please see our guide here.