Early clip-on frames were designed for two purposes. They were often used to hold reading glasses. The primary frame held distance lenses. When the user needed a reading lens, he or she simply installed the clip-on frames. The clip-on frame was also used to hold sunglass lenses. Most vintage clip-on’s were desiged for use with the round Windsor and Marshwood frames. Early clip-on’s were also made in the ovid style. The ovid style is often referred to as “P3”. Generally clip-on frames are slightly larger than the frame they are attached to.