Segment height is the part of your progressive lenses where the strength of your prescription changes. It is measured from the bottom inside of the lens to the center of your pupil when you wear the glasses. The segment height varies for each pair of glasses due to different frame sizes. Don't worry if you don't know your segment height because every frame we offer comes with a predetermined factory segment height. Just choose the value 13 in the Segment Height dropdown options and we will do the rest.