Corsets

Continuing the profiles of women's undergarments: the much-maligned corset or stays.  Both terms appear interchangeably in the period--with "stays" more common than "corset" in the preceding decades, as far as I can tell--though modern usage is more likely to style a steel-boned article as a "corset" and a corded one as "stays".

Corset, c. 1861-3
This example is shaped with curved pieces seamed together.

Corsets are widely used for women's and children's clothes of the 1860s; some men wear a similar garment dubbed a "belt".  Worn over the chemise (and possibly the drawers), the corset acts as a foundation for the other garments: it distributes the weight of petticoats and dress over the the entire torso, while simultaneously smoothing the line of the upper body and supporting the bust.  The closely-fit dresses of the 1860s tend to wrinkle and look unkempt when worn without a corset.  A corset also provides great back support for the working woman.

Corset, c. 1860
The three studs belong to a separating metal busk, which allows 
the corset to be taken on and off without fully unlacing the back.

At it's most basic, a corset is a tightly-fitted under-bodice; gores (triangular inserts) or curved seams allow the shape of the garment to follow the figure. Steel or whalebone "stays" or "bones" (examples here) keep the fabric smooth; this may also be accomplished by closely-set cords quilted into the garment. The fabric used in corsets needs to be tightly-woven and have little stretch.  A solid wood or separating metal busk supports the center-front of most corsets.  Whether it opens at the front or has a solid panel there, the corsets of the 1850s/60s almost universally lace up the back.  This does not make them "one size fits all"--rather, it is what allows the garment to be fastened sufficiently snug.  A "spring" or gap of several inches between the back edges is desirable, so that the corset is not too loose.  A very large gap is less supportive of the back.

Corset, 1840s
This corset has the shoulder straps, solid busk, and long line
of the early 1800s.  By the 1860s, shorter corsets are more common.

Stays from The Workwoman's Guide, 1838/40
Like the above, they are shaped with triangular gores over the
bust and hip, and have a solid center busk and shoulder straps. 

Pattern pieces from Godey's (1857).  Now without shoulder straps,
these stays are still shaped with gores at the bust, hip, and stomach.
The stays open in the front with a steel busk. An identical pattern 
appeared in Peterson's two years before.

The "Corset de Medici", Peterson's, 1855

Gored "French Corset" from Godey's, 1862

"Victoria Corset", Godey's, 1862

Gored Corset from Der Bazar, 1865

Curved seam corset with separate hip pieces.
Patented by Lavinia H. Foy, 1868.

This probably the time to mention that Gone with the Wind is post-period fiction, and that tight-lacing corsets is 1) universally condemned, and 2) going out of fashion by the 1860s.  Medical texts blame it for everything from scoliosis to consumption to deformed ribs to miscarriage.  This 1863 book gives the average waist measurement as 27"-29", contrasted with that of tight-lacers (23" or even 21")--earlier, in 1845, a similar article gave the same "normal" measure of 27"-29", but claimed that most women laced down to 24", and some even to 20".  As for fashion:

A very small waist is rather a deformity than a beauty. To see the shoulders cramped and squeezed together, is anything but agreeable; the figure should be easy, well developed, supple: if Nature has not made the waist small, compression cannot mend her work. Dress may do much to lessen the awkward appearance of a thick waist by clever adaptations; by the use of stays both easy and well fitting; by a little extra trimming on the shoulders which naturally makes the waist appear smaller. All this may be done without injury; no stays can answer the purpose so well as those made by a good French stay-maker, who has the art of taking a sort of model of the figure by the extreme exactness of her measurements." --The Habits of Good Society, 1859/1863

Another text gives ideal "proportions" (scaled from the Venus di Medicis):

...therefore, the waist of a person five feet three inches high should not be less than twenty-five and a quarter inches; of five feet five inches, twenty-six inches; of five feet seven inches, twenty-six and three quarter inches; of five feet eight inches, twenty seven and a quarter inches. --Medical Common Sense (1863) 

It concludes that "a very small waist is a defect rather than a beauty, and nothing can be truly beautiful which is out of proportion."