Semi rim eyeglass frames, also called “grooved” frames, only cover half the prescription lenses. They use nylon strings or liners running along the bottom of the lenses to hold the prescription lenses, so the semi rim glass frames are lighter than full rim glass frames. When the semi rim prescription glasses assembled, it need to be cut grooves into the prescription lenses edges.Therefore, the thicker prescription lenses are required for the semi rim eyeglass frames in order to not chip and crack. cheapprescriptionglasses.net has various semi rim glass frames for your choice!