When people talk about sunglasses they talk about shape: round, square, cat-eye. There is a second choice that changes the look just as much and gets far less attention: how thick the frame is. The same round lens can be a hairline of gold wire or a heavy black ring, and those two pairs say very different things about the outfit under them.
A frame is a line drawn around your eyes. A thin line is quiet and lets the face and the clothes lead. A thick line is a graphic mark: it is the first thing anyone sees, and everything else in the outfit has to share the stage with it. Neither is better. What matters is that the frame and the outfit don't both shout.
Thin frames, roughly 1 to 2 mm, are usually metal. They keep the eyes and brows visible and they take up almost no space, so the clothes lead. That makes them an easy partner for patterned or textured outfits, such as stripes, checks or a chunky knit, because there is only one loud thing in the look. The trade-off is that a thin frame can disappear against a busy or dark outfit, and a plain outfit with a hairline frame can feel a little undecided. Gold or silver metal also adds a small glint, which works like jewellery.
Between about 3 and 5 mm you get the frame most people picture when they imagine everyday sunglasses. It is visible without dominating, and it sits comfortably with both plain and tailored clothes. If you only want one pair and you wear a mix of things, this is the safe middle. Colour matters more here than at either extreme: black reads graphic, tortoiseshell reads warm and softer, and both are easy to match with leather shoes and belts.
From about 6 mm up, the frame becomes a feature of the outfit in its own right. A thick black rim next to a white tee, or a dark tailored jacket with a clean collar, looks deliberate and editorial: the sunglasses are the accessory and nothing else is competing. Put the same frame with a bold stripe or a loud print and you have two statements fighting. In that case, either choose a thinner frame or let the clothes be quiet.
Thick frames also cover more of the upper face, so they read as a mask more than thin ones do. If you like to keep your eyes visible, pick a lighter lens or a thinner rim.
Let one thing be loud. If your clothes have pattern, strong colour or a lot of texture, keep the frame thin or medium. If your clothes are plain and tonal, the frame is free to be the statement. Between those, the shape of the frame, the lens colour and your hair matter more than anything else, and you can break the rule on purpose when you want the extra impact. Try each combination in the figure above and watch the note under the drawing change.
If you already own a pair you like, try it with a different outfit before buying another. A frame that looks too heavy with a print can look just right with a plain white shirt.