It usually happens when your eye care provider measures your PD as the dual pupillary distance, which is the distance in millimeters from each eye's pupil to the center of your nose. 
It is often written such as "31/32" on your prescription.
If it looks like this, one thing you should know is that the first dual PD number "31" is always your right PD, the distance in millimeters from your right eye's pupil to the center of your nose. 
The dual PD number "32" is your left PD, which is the distance in millimeters from your left eye's pupil to the center of your nose.

You need to click "Two PD numbers" button first and select the exact dual PD numbers when filling in the prescription form.

Note: 
• Range of dual PD: 22-40mm

Or it happens when your eye care provider measures your two different types of PD, distance PD and near PD. 
It's usually written such as "62/59" or "Distance PD: 62 & Near PD: 59".

If it looks like this, the PD number you fill in varies according to how you use your eyeglasses for.
If they're used for DISTANCE, seeing things far away, your distance PD is necessary.
But if they're used for READING, seeing things up close, your near PD is necessary.
If you need multifocal lenses (progressive or bifocal) for your eyeglasses, your distance PD is essential.

Note: 
• Range of Adult PD: 54-74 mm
• Range of Child PD: 43-58 mm

You might be confused about it when you don't know well about PD measurement and prescription. 
If so, welcome to contact us at any time. We are pleased to assist you with it.