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What is your Face Width?

It is the ear to ear distance across your eyes measured in millimeters (as shown in be /low image)

What is Frame Width?

Frame width is the fundamental way to determine the fit. It is the horizontal front of the frame, measured from the furthest extended point on either side

If the frame is 132mm, then it'll work for people having 130mm to 136mm face size.
Face within -2mm and +4mm from your frame measurement and usually suitable fit
Note: You will not find this number on your glasses