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Tightlacing (also called corset training and waist training) is the practice of wearing a tightly-laced corset to achieve extreme modifications to the figure and posture. Those who practice tightlacing are called tightlacers.
The most frequent aim of tightlacing is waist reduction. Depending on the silhouette desired (hourglass, wasp waist, and pipe-stem waist are the most common), the shape of the ribcage may be altered as well. Wearing a corset can also improve the bustline, by raising the breasts upwards and shaping them, flattening the stomach, and improving posture. However, these effects are only temporary and will be lost on removing the corset. One possible permanent effect of tightlacing is a change in the shape of the ribcage. However, this can only happen with consistent years of tightlacing. 
Tightlacing has its roots in the nineteenth century, when improvements in technology allowed the mass manufacture of corsets that were stronger than before. The Victorian and Edwardian ideal of female beauty emphasised a small waist, and it was widely accepted that corsets be used to reduce the size of the natural waist.
However, it is important to note that although some nineteenth century women had waist measurements that today's tightlacers would envy, they cannot easily be classified as tightlacers.
- Firstly, women's bodies have increased in size since the nineteenth century, so waist measurements that seem small today might not have been considered so by Victorians.
- Secondly, as corset wearing was the norm in the nineteenth century it is possible that women tolerated proportionally greater reductions as a matter of course, without thinking of themselves as tightlacers.
There were also factors that put women off tightlacing. In the late years of the Victorian era, medical reports and rumors claimed that tightlacing was fatally detrimental to health (see Victorian dress reform). Women with small waists were condemned for their vanity and being slaves to fashion, and it was frequently claimed that too small a waist was actually ugly, rather than beautiful.
Modern tightlacing is a minority interest, often associated with fetishistic interest in the corset and BDSM. The majority of tightlacers are women, although some men do tightlace - corsetier Mr Pearl has a nineteen-inch waist.
Tightlacers typically wear a corset for at least 12 hours a day, every day, when they are most active, although some tightlacers wear corsets for up to 23 hours a day, taking the corset off only in order to bathe.
Tightlacers usually have a partner, called a trainer, to help and support them. However, it is possible—although difficult—for somebody to tightlace without a partner. (Tightlacers are often—but not necessarily—in a sexual and/or loving relationship with their trainer.)
A partner might take on any of the following tasks:
- help the tightlacer put on and take off the corset
- help him or her follow through with the training schedule
- monitor the tightlacer's health
- monitor body changes and keep a log 
Contemporary tightlacers claim that tightlacing does not adversely affect the health, as was believed in the later Victorian era. Certainly, there are no contemporary medical sources condemning tightlacing, and the continued good health of modern day extreme tightlacers would seem to demonstrate that the practice is not dangerous—if properly done.
A safe training routine begins with the use of a well-fitted corset (most serious tightlacers have at least one custom–tailored corset) and very gradual decreases in the waist circumference—it will take a tightlacer at least eight months (and possibly up to eighteen) to achieve reductions of more than a few inches. Attempting large reductions in waist circumference before properly accustomed to torso constriction can cause extreme discomfort and as digestive and respiratory volume is decreased, possibly difficulties with indigestion and respiration.
Some tightlacers call the corsets they wear training corsets.
- Staylace.com, the website of the Long Island Staylace Association (LISA) has some further information on the medical considerations and effects of tightlacing.
- THE CORSET, questions of pressure and displacement. A medical report from 1887.
Valerie Steele, The Corset: A Cultural History. Yale University Press, 2001, ISBN 0300099533.