A Short History of the Corset

In the 18th century, stays are definitely underwear. Only in case of the Robe à l'Allemande, the stiff bodice survived until about 1730, in case of the French court robe even longer. The shape of stays is not much different from that of the 17th century: Conical, pressing the breast up and together, with tabs over the hips. The tabs are formed by cuts from the lower edge up to the waistline that spread when the stays are worn, giving the hips room. They prevent the waistband of the skirt from crawling under the stays, and the waistline of the stays from digging into the flesh.
There are stays that lace at the back (Diderot calls them corps fermé, closed stays) and those that lace across a stiff stomacher in front (corps ouvert, or open stays). Examples that lace both back and front (but not over a stomacher) are quite rare. Stays that lace in front only are even rarer and so far only known to me from the region of Southern Germany. In all these cases, spiral lacing is used.
Although 18th century stays were not meant to be seen, they are often quite decorative, with finely stitched tunnels for the boning, precious silk brocade and possibly gold trim. The inside, on the other hand, usually looks downright sloppy, even in outwardly fine stays.
The basic shape of stays didn't change the whole century long. Towards the end, around 1790, when dress waists begin to wander upwards, the stays become slightly shorter. Since paniers were not worn anymore, the skirt is supported by small pads sewn to the tabs. At the same time, physicians make themselves heard, warning against the harm done by tight-lacing. While lacing wasn't usually overdone as much as one century later, it often started earlier: It started with tightly wrapping babies and included children's corsets, forcing the still soft skeleton into a fashionable shape.