The measurements on the inside of the frame represent the lens width (51 mm), bridge width (18 mm), and temple length (140 mm). If you have a favorite pair of glasses, you can look at this measurement to figure out which of our pairs will fit you the best.
Lens width51 mm
The width of one single lens.
Bridge width18 mm
The measurement between the lenses.
Temple length140 mm
The measurement from the front of the frame to the back of the arms.