Segment height is the distance, in millimeters, from the bottom of the lens to the top line of a lined bifocal or the beginning of the progressive addition on a progressive lens. Segment height does not apply to single-vision lenses and should be left blank if you are ordering single vision. The segment height is measured differently depending on whether you get progressives or a lined bifocal. Either way, you will need the frame to be able to measure segment height because you can only measure it while the glasses are on your head. That means that you'll have to order the frame without lenses first (click the "Buy Frame Only" button on the item description page), then send it back to have lenses put in after the segment height has been measured.
Here's how to measure your segment height: Put the glasses on your face and adjust them until they sit comfortably. Have a friend use a metric ruler to measure from the bottom of the lens pocket (the top of the bottom rim of the frame) to the center of your pupil (while you're looking straight ahead) for progressives, or to the top of your bottom eyelid for lined (FT) bifocals.
Segment height is more important for progressives than it is for lined (FT) bifocals, because for lined bifocals we can use a standard measurement if you leave this section blank, and you will most likely be comfortable with it. Progressives require more precision because of the complexity of the lens. If you have any questions or problems, feel free to call us at (888)-440-9797.