The Ballon Bleu is the round one, and it is a much younger idea than the Tank. Hodinkee says it was introduced in 2007 and is available from 26mm to 42mm, so the two sizes here, 28mm and 33mm, sit at the small end of that range. Cartier describes a doubly convex design, with an arch that protects its blue cabochon, and on its product pages the crown is a fluted crown set with a cabochon synthetic spinel. That protected crown is the detail I look at first, because it is what keeps a round case from reading like a plain circle.

Start with the six 28mm quartz watches. Four are plain stainless steel listings of the same Ballon Bleu de Cartier. Then there is the 28mm with a diamond bezel and the 28mm with diamond markers, both stainless steel. The plain ones are the quietest in the grid, and I would wear any of them with a white shirt and jeans. The diamond bezel is the dressier one, so I would keep the rest of the outfit very simple around it.

The two 33mm watches are the only automatics here. An automatic mechanical movement is powered by the motion of the wearer's wrist, while quartz is battery-powered. That is the practical difference between the 33mm watches and everything else in this grid, and it is worth knowing before you choose, because a watch that runs on wrist motion likes to be worn most days.

My fitting-room test is the cuff. Put the watch on over a shirt sleeve, a knit and a trench, and see whether the round case catches. A 28mm tends to slide under most cuffs. The 33mm watches will show more, which is the point if you want the watch to be noticed. Every one of these is pre-owned and a single piece, so go through the listing photos closely before you pick one.

This is the group I find the most fun, because the shapes stop being rectangles. It also has a split I want to be clear about. Two pieces are properly small: the Santos Demoiselle in stainless steel at 20mm and the Mini Tonneau Lanier in yellow gold at 16mm. The other six are mid-size watches at 35 to 36mm that many women wear, so I would not call them small.

Start with the Santos. Cartier says that in 1904 Louis Cartier granted the aviator Alberto Santos Dumont's wish to tell time while flying, producing one of the first ever wristwatches. The Demoiselle is the small, quiet version of that idea. The Tonneau came two years later, in 1906, and tonneau is the French word for barrel, which is the shape of the case. It was later made in a smaller size for women in the 1980s and 1990s, some with diamonds on the bezel, so a 16mm Mini Tonneau fits that line well.

Now the mid-size rounds. Hodinkee reports Cartier launched the Cle de Cartier at SIHH in January 2015, named for its key-like oblong winding crown set into the side of the case. Here it is an automatic at 35mm three times: two in stainless steel and one in stainless steel and rose gold. The Pasha C comes in stainless steel at 35mm, and as a GMT, also in stainless steel at 35mm. The Rake says the Pasha de Cartier arrived in 1985, designed with Gerald Genta, a round watch that stood out among Cartier's ovals and rectangles, with a screw-down crown cap on a small chain. The Ronde Solo de Cartier in stainless steel at 36mm is a quartz and the plainest of the group.

What I would skip: do not choose a mid-size watch on the idea that it will disappear under a cuff. Try the 35mm and 36mm watches next to the 16mm and 20mm ones, and choose between a watch you notice and one you find by accident.

Here is how I would choose. First, decide on shape: a rectangle, a round, or the odd one. Then use your wrist bone as the ruler, because the same number in millimeters can look quite different on a Tank than on a round case. Try the watch with the sleeves you actually wear.

Second, decide quartz or automatic. The first is battery-powered and the second runs on the motion of your wrist, and both appear in this edit. Third, decide how much detail you want: a plain dial, diamond markers, a diamond bezel, or two metals together.

Last, remember that every watch here is pre-owned and one single piece. If a listing speaks to you, read it, look at every photo, and go with the one you keep coming back to. If you want to keep browsing, I also put together Cartier Panthère Watch and Gucci Watch for Women.