Typically, corsets are constructed of a flexible material (like cloth, satin, brocade, patent leather, or leather) stiffened with boning (also called ribs or stays) inserted into channels in the cloth or leather. In the 19th century, bones of elephant, moose, and whale were favored for the boning.
Corsets are held together by lacing, usually (though not always) at the back. Tightening or loosening the lacing produces corresponding changes in the firmness of the corset. Depending on the desired effect and time period, corsets can be laced from the top down, from the bottom up, or both up from the bottom and down from the top, using two laces that meet in the middle. It is difficult—although not impossible—for a back-laced corset-wearer to do his or her own lacing.
Plastic boning is now the most commonly used material for lightweight corsets and the majority of inexpensive and ultimately lesser quality corsets, referred to as fashion corsets. Waist reduction is not possible with a fashion corset. If you decide on a fashion corset for your costume or outfit, just remember that since they have plastic boning, they will not offer the same support as a steel boned corset. The point to remember is, with a plastic boned corset, your body will shape your corset, your corset will not shape your body.
Flat or spiral steel boning is preferred for stronger corsets and generally better quality corset, too. High quality steel boned corsets will keep their shape over time even if you want to wear them all night long, whereas acrylic (plastic) boned corsets will not. The steel bones make for a smooth, streamlined shape without any lumps or bumps and they really will change the figure into a nice hour glass if laced tightly. Most wearers can expect a four or five inch waist reduction.
Don't be put off by thinking steel would be uncomfortable either. As any connoisseur will tell you, well-made steel boned corsets offer an outstanding level of support and that means comfort too. In fact, when wearing a plastic boned corset, I can tell you that it is much less comfortable than a steel boned corset, because the boning is not meant to hold you in. Because of this, the boning buckles and pinches your skin. Steel boned corsets hold you in, prevent pinching, and will pull waistlines in by 4-5" and will push and lift the cleavage if desired. These garments are fully adjustable at the back so you can tension according to comfort or control. Good Steel Boned Corsets have a minimum of 14 steel bones and up to 24 steel bones for ultimate control, 100% cotton lining for comfort and removable modesty panels allowing you to cover your back or wear the corset open. Authentic corsets have steel busks (unless otherwise noted), a metal lacing bone, and several layers of fabric for strength.
Waist training corsets have the most slimming and shaping capability of any other. The potential waist reduction is around seven inches for most wearers. I strongly recommend starting with less tight lacing and working your way up (or in) to a sexy hourglass over a period of time. In order to achieve a waist reduction of six or seven inches, a corset needs to be very well constructed. 24 spiral steel bones are used to make sure to deliver the results desired. Wearing waist training garments over a long period of time can change your body shape, and not just while you're actually wearing the corset. Permanent changes are possible. In fact, for some people a long-term change in body shape is the ultimate goal. If you're one of them, remember that anything that changes your body shape permanently is a pretty big deal. Talk to a medical professional before starting this kind of program.
In contrast, a girdle is usually made of elasticized fabric, without boning. Common brands of “shapewear” such as girdles, garter belts, and waist cinchers are Rago, Spanx, and Hanes. In my opinion, Rago is the best shapewear in the market right now offering the most shaping and contouring of any elastic, boneless shapewear. These products will be comfortable, slimming, provide some shaping, and reduce the “muffin top”, but do not offer much in the way of inches in waist reduction.
I hope this serves as a helpful buying guide. If you have any questions regarding what different types of boning look like or need more specific information about how a corset is constructed, you can search the web and find lots of Wikipedia articles, pictures, and videos to further your education. Thank you for reading!