These accessories usually attach to glasses by two or four small clamps: two reaching over the top of the frame and sometimes two more on the bottom. The quality of this attachment system is important to prevent scratching your lenses. Neither the clip nor the tinted lenses themselves should touch the prescription lenses - this will invariably lead to scratches. Some models are attached by clips or magnets that don't mar the lenses but may fall off more easily.

Some eyeglass frames are sold with matching clip-ons designed for an exact fit. Some can be purchased through a different manufacturer but may fit your frames quite well. Custom-made clip-ons are also available. The fit is critical to protecting your eyeglass lenses from scratches.

Flip-ups are not attached at the bottom. They usually hang from two clips on the upper part of the frame/lenses, floating freely over the prescription lenses. A spring system enables the wearer to flip them up when indoors and flip them down when in bright sunlight. The quality of the spring system is the most important variable; these clips usually contact the lenses and will scratch them.

Clip-ons and flip-ups come in a variety of materials and with a range of treatments-polycarbonate, solid tints, mirror coatings, polarized lenses, UV filters, anti-reflective coatings-just like sunglass lenses.

Flip-ups may be available for some sports eyewear. For example, magnifying lenses can be used in a flip-up lens for wear by a fisherman when tying flies.

Eyeglass Cases
Eyeglass cases are a crucial part of your care regime, for glasses, sunglasses, clip-ons, and flip-ups - they protect glasses from damage.

Cases may be made of hard plastic, soft vinyl, wood, leather, aluminum, cloth or other materials. Designs range from soft-bodied fabrics with tab closures and decorative prints, to hard, burnished metals with soft velour linings and spring hinges. Some designers use exotic covers like python and ostrich skin. Many optical shops give complimentary cases with the purchase of eyeglasses.

Cases are especially useful for people who frequent workshops, laboratories, construction zones or industrial areas where flecks of chemicals and glue or wood shavings can damage unshielded eyeglasses. Of course, in these environments, wearers need to remove their glasses or sunglasses and wear more appropriate protective eyewear.

Boaters and outdoor enthusiasts can purchase crush-resistant cases with waterproof seals and soft interior padding, some of which even float. Some cases accommodate wrap-around eyewear. For ease of carry, some cases clip on to your belt or pocket.