Eyewear Glossary

Buying glasses online means reading a lot of words that nobody explains. Here is what the terms on our product pages actually mean, written so you can tell whether a frame will work for you before it arrives.

Jump to: A · B · C · E · G · H · L · M · N · O · P · R · S · T · U

A

Acetate

A plant-based plastic cut and polished from solid sheets rather than poured into a mold. Color runs all the way through the material instead of sitting on the surface, which is why acetate frames hold their tortoise and marbled patterns as they wear. It also warms to the touch and can be reshaped by heat, so an optician can adjust the fit.

Anti-Reflective Coating

A thin layer applied to the lens that cuts glare bouncing off the back and front surfaces. On clear lenses it stops the reflections that hide your eyes in photos and on video calls. On sunglasses it is usually applied to the inside only, so light coming from behind you does not bounce back into your eye.

Aviator

A teardrop lens with a thin metal rim, a double or triple bridge, and a downward slope at the outer edge. The shape was built for pilots to cover the full range of the eye while looking down at instruments, which is why the lens sits low on the cheek. Works on most face shapes, and the metal construction keeps a large lens light. Shop aviators.

B

Base Curve

How steeply the lens curves across its front, measured in numbers from about 2 to 10. A low base curve is nearly flat and suits classic square and rectangular frames. A high base curve, usually 8 or above, wraps toward the temples and is what sport and cycling frames use to block light from the side.

Beta-Titanium

A titanium alloy with more flex than pure titanium. It bends further before it takes a set, which makes it the usual choice for very thin temples and rimless designs where a rigid metal would snap.

Bio-Acetate

Acetate made with a higher share of plant-derived content and without petroleum-based plasticizers. It looks and behaves like standard acetate on your face. You will see it called out in the lens and frame codes on some of our styles.

Bridge

The section of the frame that crosses your nose and joins the two lenses. Bridge width is the second number in a frame's size and is measured in millimeters between the lenses. If frames slide down your nose, the bridge is too wide; if they sit high and leave marks, it is too narrow.

Butterfly

A lens that sweeps up and out at the outer corners, wider at the top than the bottom. It reads as dramatic and gives good coverage without the bulk of a true oversize frame. Flatters narrow and heart-shaped faces because the width sits above the cheekbone. Shop butterfly frames.

C

Cat Eye

A frame that lifts at the outer edges of the top rim, with the point sitting above the temple line. The upward angle draws the eye up and out across the face. It has been in and out of fashion since the 1950s and has never really left. Shop cat eye frames.

Category (Lens Category)

A rating from 0 to 4 describing how much visible light a tint blocks. Category 0 is nearly clear, category 2 and 3 cover most everyday sunglasses, and category 4 is very dark and not legal for driving in many places. Category has nothing to do with UV protection, which is measured separately.

CR-39

A lightweight plastic lens material with excellent optical clarity, long used for prescription lenses before polycarbonate arrived. It scratches more easily than glass and is less impact-resistant than polycarbonate, but the optics are very clean.

E

End Piece

The small section at the outer edge of the front where the hinge attaches. On acetate frames it is often thickened to carry the hinge, and on metal frames it is where the temple folds in. A broken end piece is one of the harder repairs, which is why frames should be taken off with two hands.

G

G15

A grey-green lens tint originally developed for military aviation. It cuts brightness while keeping color relationships close to accurate, so reds and greens still look like themselves. A safe all-purpose tint for driving and general daylight.

Gradient Lens

A lens tinted dark at the top and fading lighter toward the bottom. It shades against overhead sun while leaving the lower part clear enough to read a menu or a dashboard. A double gradient is dark at both top and bottom and lighter through the middle, which suits bright environments with light bouncing off water or sand.

H

Hinge

The joint where the temple meets the front. A barrel hinge uses interlocking loops and a screw. A spring hinge adds a small internal spring that lets the temple flex outward past its normal stop, which helps on wider faces and takes stress off the frame. Some designers use screwless hinges built from the frame material itself.

L

Lens Width

The horizontal width of one lens at its widest point, in millimeters. It is the first number in a frame's size and the single most useful figure for judging whether a frame will suit your face. Measure a pair you already like and compare rather than guessing.

Low Bridge Fit

A frame built for a nose bridge that sits level with or below the pupils, often with wider nose pads and more space at the cheek. It solves the two most common fit complaints at once: glasses that slide down, and frames that rest on your cheeks when you smile. Also sold as alternative fit or universal fit.

M

Mirrored Lens

A reflective coating applied to the front of the lens that bounces additional light away before it reaches the tint underneath. It cuts glare in very bright conditions and hides the eyes from the outside. Mirror coatings sit on the surface, so they show fingerprints and scratches more readily than a plain tint.

Monel

A nickel and copper alloy used widely in metal frames. It is corrosion-resistant, easy to adjust, and inexpensive relative to titanium. People with nickel sensitivity should look to titanium or stainless steel instead.

N

Nose Pads

The small pads that carry the frame's weight on your nose. Adjustable pads sit on flexible arms and can be moved in or out to raise, lower, or level the frame, which is why metal frames tend to fit a wider range of faces. Acetate frames usually have a fixed molded bridge instead, so the shape has to suit you from the start.

O

Oversize

A frame noticeably larger than the wearer's face, usually with a lens width above 55mm. Beyond the look, the extra size means more coverage from sun and wind. Check the temple length as well as the lens width, since a large front on short temples will pinch. Shop oversize frames.

P

Pantos

A rounded frame that is slightly wider than it is tall, with a gentle flat section along the top rim. It is softer than a true round and less severe than a square, which is why so many heritage optical houses build their signature model on this shape.

Photochromic

Lenses that darken in ultraviolet light and clear again indoors. The reaction depends on UV rather than brightness, so most photochromic lenses stay light inside a car, where the windshield already blocks UV. Best thought of as convenient rather than as a replacement for real sunglasses.

Polarized

A filter built into the lens that blocks light waves traveling in one direction, which is the direction glare takes when it reflects off water, snow, wet roads, and car hoods. The effect is dramatic near water and while driving. The tradeoff is that polarization can make LCD screens, including some dashboards and phone displays, look dark or patchy at certain angles.

Polycarbonate

A lightweight, highly impact-resistant lens material that blocks UV on its own without an added coating. It is the standard for sport eyewear and children's glasses. It scratches more easily than glass, so a hard coat is usually applied.

R

Rectangle

A frame clearly wider than it is tall, with squared corners. The horizontal line adds width to a long or oval face and reads as conservative in most contexts, which is why it dominates men's optical.

Rimless and Semi-Rimless

Rimless frames hold the lenses with mounts drilled directly into the lens, with no surrounding rim. Semi-rimless, sometimes called half-rim, carries a rim across the top only, with nylon cord holding the lower edge. Both minimize the visual weight of the frame; both put more stress on the lens, so material choice matters.

Round

A circular or near-circular lens. It softens strong jawlines and angular faces, and depending on size it can read either as an intellectual classic or as a statement. Shop round frames.

S

Square

A frame with straight top and bottom rims and defined corners, roughly as tall as it is wide. It adds structure to round and oval faces. Softened versions, often listed as rounded square or soft square, keep the structure without the hard corners. Shop square frames.

T

Temple

The arm running from the front of the frame to behind your ear. Temple length is the third number in a frame's size, typically between 135mm and 150mm. Too short and the frame grips your head; too long and it slides forward.

Titanium

A metal that is lighter than steel, stronger for its weight, corrosion-resistant, and hypoallergenic. It is the standard for premium metal frames and for anyone with a nickel sensitivity. It costs more than monel because it is harder to machine and weld.

U

UV400

A rating meaning the lens blocks all light below 400 nanometers, which covers both UVA and UVB. It is the figure to look for, and it is unrelated to how dark a lens is. A dark lens without UV protection is worse than no sunglasses at all, because the tint opens your pupil and lets more UV through.

Reading a frame size

Most frames carry three numbers on the inside of a temple, printed as something like 52-18-145. That is lens width, bridge width, and temple length, all in millimeters. If you have a pair that fits, those three numbers are more reliable than any face-shape guide.

Shopping the other side of the store? See our intimates and lingerie glossary.