Square and rectangular frames do the opposite job. Where the butterfly lifts, a square steadies. This second group is the one I reach for with a trench, a tailored blazer or a gray knit set, because the straight top line reads as precise rather than playful.

Look at the temples. Marcolin describes the elegant metal T on its Tom Ford frames as an unmistakable hallmark of the brand, and Tom Ford's own site describes an icon rectangular style as having a shiny metal T logo on the temples. It is a small, neat detail, and it is the reason a plain black square can still look like it came from somewhere in particular. Turn the pair sideways in your hand and check that it sits clean and even.

For the squares, the Vincent Square at 56mm, the Morgan Square at 55mm and the Emma Square at 51mm are the named pairs, and the Emma is the smallest and the most delicate on a narrower face. The unnamed Square pairs, at 53mm, 54mm and 55mm, are the plain ones, and I would call them the easiest first pair for anyone who has never owned a designer sunglass. The Jake Rectangular at 54mm shows up twice, and the Penn Rectangular at 53mm is a close cousin. The two other Rectangular pairs, at 53mm and 64mm, are the extremes: the 64mm is the widest pair in this group, and I would wear it with a very simple outfit and nothing else going on.

The practical test here is the width. Hold the frame against your face with the arms open: the outer edge should sit about level with the widest part of your face, not poke out past it. If it does, go smaller.

This last group is the softer one: round, oval, pilot and navigator. Rounded lines are friendlier on a face that already has strong angles, and I like them with denim, a slip dress, a camel coat and not much jewelry.

The pilots and the aviators are the classic travel pairs. The Clark Aviator comes in at 59mm, and the Reggie Brow Bar Aviator at 61mm carries, as its name says, a bar across the top, which gives it a straighter, sterner line. The Bronson Pilot at 60mm and the plain Pilot at 55mm are the closest in feel. For the navigators, the Rosco at 58mm, the Eliott at 59mm and the Stavros at 63mm sit between a square and a pilot, and the Stavros is the biggest of the three.

The ovals are the gentler end. The Iggy Oval at 49mm is the smallest pair in the whole edit, a neat little frame, and the plain Oval at 52mm and the two Chloe Oval pairs at 52mm sit just above it. The Icon Collection Round at 57mm is the one true circle here, and I would wear it with a messy bun and a big knit.

What I would skip: a very large frame, like the 63mm Stavros, if you plan to wear it all day on a small face. It is a lovely pair for a beach or a drive, but it will slide down your nose in a long afternoon. I would also skip buying on shade alone. The American Academy of Ophthalmology says darker lenses do not block more UV, and that the color and darkness of a lens say nothing about its protection.

Here is how I would choose one pair. Decide the shape first, then the size, then the color. If you wear a lot of tailoring, start with the squares. If your outfits are soft and slouchy, the butterfly or the oval will give them a lift or a finish.

Then check the protection, not the look. The American Academy of Ophthalmology says to choose sunglasses that block 99 to 100 percent of both UVA and UVB radiation, and that a label of 100 percent UV400 means the same thing. I would read the label on any pair before I decided, and I would not assume anything from the lens color.

A last word on screens. The same group says it does not recommend blue light-blocking glasses for computer use, because there is no scientific evidence that blue light from screens damages the eyes. So these are for the sun, and that is where I would wear them. Try them on in daylight, at a window if you can, and keep the pair that feels like you. If you want to keep browsing, I also put together Versace Glasses for Women and Glasses That Do the Talking.