Visit the “Post Series” page for the full series.
Movies frequently present medieval women wearing (or being upset about) their corsets. Clearly, these costume designers have never bothered to look at the flowing clothing in medieval art, or if they have, they misinterpreted it.
Before the twelfth century, women generally wore very loose gowns with long sleeves with so much fabric that in art, their clothing looks several sizes too large.  As the twelfth century drew closer, dresses became fitted to the body on top by means of lacing up the back or under the arms, leaving sleeves and the skirt still very loose. 
In the twelfth century, “The outstanding new features of women’s…costumes were the accent on vertical lines and the sheer supple texture of the fabric which made those lines possible.”  Standard was the bliaut, a two-piece dress with a tight-fitting bodice, long, wide sleeves, and a full skirt.  The word “corset” used in this period refers to a girdle (girdle meaning a type of wide sash). Such “corsets” could be several yards long and could be wrapped tightly around the waist in a constricting manner , but this is nothing like the tight-lacing of later periods.
In the thirteenth century, clothing became simpler; women frequently wore loose-fitting gowns with long, narrow belts.  The cyclas, an outer garment that looked something like a long rectangle with a hole for the head, was worn by both genders. It may or may not have been sewn along the sides, as fashion dictated. 
In the fourteenth century, women wore the cotehardie and the sideless gown. A cotehardie consisted of a dress with a flaring skirt, long, tight sleeves, a wide, low neckline, and fastenings down the back.  The sideless gown was similar to a cyclas, but it was sewn on the sides from the hip down while armholes were cut in deep curves.  Ornate belts were worn low on the hips.  Long tippets were often attached to the sleeves , giving us the usual picture of medieval noblewomen. Seams were not used to a great degree for shape—godets were used to ensure a tight bodice with a flaring skirt , but again, this is not the type of dress worn over a tight-fitting corset.
In the fifteenth century, we are nearing the influence of what may be better termed “Renaissance fashions.” Still, though cuts of dresses, materials, and headdresses became more elaborate and waistlines rose, the basic shape of the dress was similar. In the end of the sixteenth century, after 1580, we start to see enormous skirts with massive amounts of construction to hold their shape , and here do we finally see the emergence of the corset. 
Why the confusion? I suspect that much of it has to do with Renaissance festivals and associating corsets with an exciting, sexy past. Some may also be confusing the lacing used to make dresses fit properly with the tight lacing of later centuries. Before the invention of elastic, lacing was simply the best way to make clothing fit. Not terribly exciting, but there you have it.
- Payne, Blanche. History of Costume: From the Ancient Egyptians to the Twentieth Century, (New York: Harper and Row, 1965), 149.
- Ibid., 152.
- Ibid., 164.
- Ibid., 165.
- Ibid., 165-66.
- Ibid., 174.
- Ibid., 189.
- Ibid., 190.
- Ibid., 192.
- Ibid., 311.
- Ibid., 313.