When people think of 16th century dress, the first thing that comes to mind is the corset. The corset represents a fundamental shift in the concept of clothing and tailoring; instead of shaping clothes to the body, as had been done throughout the Middle Ages and Renaissance, the body began to conform to the fashionable shape of the clothing worn.
There are several myths about wearing corsets, many of which spring from Victorian corsetry rather than Elizabethan. In the 16th century, the corset was not meant to draw in the waist and create an hourglass figure; rather, it was designed to mold the torso into a cylindrical shape, and to flatten and raise the bustline. There is one 16th century reference to a small waist being fashionable, but on the whole it was a fashionably flat-torsoed shape, rather than a tiny waist, that the corset was designed to acheive.
Another common myth revolves around the horrible discomfort of corsets. This, too, stems from the tightly-laced waists of the 19th century; the corset worn in Elizabethan England, when fitted and laced correctly, is quite comfortable. Some well-endowed women consider then more comfortable then modern underwire bras, and many people with back problems have remarked how much a boned-tab Elizabethan corset feels like a supportive back brace.
As the corset was hidden underneath the other layers of dress in the 16th century, finding out about it is difficult. Up to the 1520s, the raised and slightly rounded shape of the fashionable gown could be achieved by a well-fitted kirtle. A German woodcarving of 1520 shows a woman wearing a gown with a definite crease and fold in the fabric under the bust. In Holbein's sketches of the 1520s and his portraits of the 1530s, however, stiffening is definitely required.
One possible method for creating this flattened bosom is that the Tudor bodices and stomachers were stiffened with buckram (glue-stiffened canvas) to achieve the fashionably flat shape. There is a reference in a Tudor wardrobe account to "buckram for stiffening bodices". Looked at from a practical standpoint, however, it saves time and labor to have one stiffened undergarment to wear under several gowns then to stiffen every gown individually. Having an undergarment to take the strain of shaping the body also helps to extend the life of the outer gown. In addition, tightly-fitted and supportive undergowns worn underneath a decorative outer garments were found through Europe for the entirity of the preceding century; it is only natural that this established trend should have continued.