Renaissance and Baroque: The origins of the corset are unknown. From the early 16th century, corset-shaped cages of iron are preserved*, but it's almost certain that they had nothing to do with normal clothing. Theories run from early fetish accessories to brute attempts at orthopedics. Judging from contemporary depictions, stiffened bodices must have been worn around 1530 because the straight, conical line of the torso seen e.g. in portraits of Venetian ladies or Eleanora di Toledo could not have been achieved otherwise. The neckline is relatively high and the chest pressed flat rather than pushed up.
18th century: 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.
Late Georgian: From 1794, the waist moved higher and arrived just under the bust around 1796. A new kind of corset is needed: The torso, hidden under flowing muslin, doesn’t need shaping anymore. The breasts still need lifting, but they’re supposed to stay apart. To achieve this, cups are employed for the first time. The busk, which in the 17th