Between prom dresses and bridesmaids dresses, we’ve seen a lot of really terrible gowns lately. What is Jessica McClintock doing to us? But it’s worse than you know—there’s a lot of weird science that goes into tacky formal dresses. You know about the engineering marvels of the Curiosity Rover, but here’s the truth about fancy gowns.

The Materials In The Fabric

Sometimes when you ask what a dress is made of, you’ll get told that it’s velvet, or satin, or lace. That’s not what it’s made of—that’s how it’s put together. Most of the fabrics down below can be made with anything from silk to rayon, and most are made with a combination of a few materials. Let’s cover the actual stuff of the dress, before we talk about how it’s put together.

If the fabric of your formal wear is natural, you probably already know what it is. The big three natural fabrics are silk, cotton, and linen. Silk comes out of a silk worm’s ass, cotton gets picked off a cotton plant in tufts, and linen is made from the flax plant. (Though the word “linen” will can be a bit of a cheat. Linen is expensive, so most fabrics with a basket-like weave, otherwise known as a “plain weave” are referred to as linens. That’s why the “linens” section of a department store is filled with cotton or synthetic fiber.)

In formal wear, the human-made fabrics you’re most likely to encounter are nylon, polyester, and rayon. Of the three, rayon is the most high-end. It was developed all the way back in 1855, as a cheaper alternative to silk. It’s the only one made from natural materials—even though those materials are cellulose (wood pulp) dissolved by anything from acetic acid salts to cuprammonium (copper) salts. Sometimes companies will hide that there’s rayon in their clothes by calling it viscose, which is the name it was originally given when it was patented.

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Polyester and nylon are both synthetic fabrics. Both are made of long chains of polymers. Both are often made with dicarboxylic acid. Both are desperately unfashionable, but are in everything. Nylon, as a fabric, will still show up on labels a lot. (As Yeoman Rand shows us, even hundreds of years into the future, people will need nylon.) Polyester is laying low by acquiring nicknames,like “China silk,” “Shantung silk,” or “Poly silk,” based on what it’s supposed to look like rather than what it is.

Putting the Materials Together

Let’s start with the underpinnings. See those rainbow-colored ruffles propping up that monstrous pink skirt? Those are tulle. Tulle netting probably originated in France, in a lace-making city of the same name but since then tulle the fabric has moved away from Tulle the city. (Unlike Chantilly lace, which is still lace that comes from the Chantilly region of France.) Up close, tulle has a roughly hexagonal pattern because it’s more of a net than a woven fabric. Some skirts are made entirely of tulle, which can be made softer and finer, and when layered gives a sort of soft, depthless look to a dress. Wedding veils are made of high-quality tulle. Mostly, though, starched or stiffened tulle stays on the underside of skirts. Whenever you see some poor woman with a huge skirt, looking like an old-fashioned wedding cake topper and rustling every time she so much as shifts in her chair, tulle is probably to blame.

Chiffon, like tulle, is slightly see-through, because the threads are widely spaced. (If there’s one thing I learned from researching this, it’s that when we get formal, we like clothes to be just a bit diaphanous.) Unlike tulle, the threads aren’t twisted around each other. They’re made with that plain basket-like weave we mentioned earlier. The threads that are held tightly in place by the loom are the warp threads, and the ones that are threaded through, over and under, are the weft threads. What makes chiffon slightly different than other diaphanous fabrics is the threads themselves are tightly twisted. This puts a little tension in them that causes the fabric to pucker, just slightly, and adds texture to a dress. The amount of twist and the way the fabric is woven together will give different dresses different looks. The main thing you need to do with chiffon is use enough of it. Enough layers, and you get the powerful khaleesi on the left. Not enough, and you get the overexposed khaleesi on the right. Chiffon is the sneaky voyeur of the fabric world. Too many people have fallen prey to it.

Most of the fabric here could be made of nearly any material, but satin and sateen buck the trend. Both are made the same way. Remember the warp and weft threads? Instead of threading the weft threads in and out in a simple grid pattern, satin and sateen float at least four different weft threads over one warp thread. This means that the material has a shine to it, but it also means that the slightest fraying is noticeable.

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Some fibers are long continuous filaments. These can be silk, or artificial fabrics like nylon or polyester. Other fibers, like cotton or yarn, are made of tiny threads twisted together. Filaments are satin, twisted threads are sateen. Sateen isn’t the prestige material because it isn’t as naturally shiny as satin. To correct that, sateen fibers are mercerized. This process, thankfully, has nothing to do with mercury—it was invented by, and got its name from, John Mercer, and involves treating the fabrics with sodium hydroxide, or some other chemical, to make them swell up. Fashion-wise, neither of these fabrics are inherently problematic. The thing is, they can be very dramatic. And some people lay drama on a little too thick.

There’s not much to say about taffeta. It’s silk in a plain weave. The only difference between it and any other kind of silk is it’s woven to be a bit more stiff and shape-holding. It’s popular for corsets and for formal, heavy dresses, so it has an old-fashioned quality to it. If you want some high-end cosplay, go get yourself some taffeta. If you don’t want to sweat through your super-expensive clothes, maybe go with something else. These days, you’ll see it on portraits, like this one of Madame de Pompadour, who was so fond of taffeta that she got a kind of patterned taffeta named after her.

Organdy and organza are another example of the diaphanous. They’re also plain weaves. They’re also wide-meshes. Unlike chiffon, they aren’t twisted enough to have a real texture to them. Organdy is cotton or nylon. Organza is silk or rayon. While chiffon has, for the most part, fallen out of favor, organza and organdy are beloved. They are beloved by brides, bridesmaids, and by designers. Sometimes, that is a good thing.

Other times . . . not so much.

And then there’s velvet and it’s evil twin velour. To get the difference between velvet and velour, both of which can be made with nearly any type of material, we have to look at the difference between weaving and knitting. Both fabrics have a “pile,” to them, meaning that, on both of the materials, little loops of thread are created on the good side of the fabric. The loops are then cut, giving the fabric its fuzziness.

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To weave a piece of cloth, as we’ve said, warp threads are stretched one way, and weft threads weave over and under those warp threads. Knitted fabrics aren’t interlocking grids of threads, they’re threads that have been looped together—just like the loops that make a sweater or a scarf, but finer. The looped fabric has a lot more give to it, because the loops are easier to tug, temporarily, out of shape. This is why velvet is for formal dresses, and velour is, for the most part, for bathrobes, sheets, and snazzy-looking work-out wear. Which isn’t to say that bathrobes can’t be formal. We know one guy who makes them work.

Bam.