Suit manufacturers produce for the average body, so for those who are close to that average, off-the-shelf suits fit well. However, the asymmetrical, extremely thin or fat, or otherwise unusually-shaped people either need their suit tailored or must change their posture to compensate, for example bend to one side or pull their stomach in. A compensating posture is often difficult to sustain for more than a few minutes with muscle tension alone. Even a not particularly strenuous posture is forgotten and omitted while doing other activities. One time-honoured solution for keeping a posture is to use mechanical devices to assist – the corset is an old example. A more modern instance was used by Chinese soldiers marching on parade: they put needles in their collars with points toward their neck, as a reminder of the correct head posture.
Someone whose spine is too curved backward or whose shoulders are too far back can compensate by deliberately slouching his posture forward, and thus fit in an off-the-shelf suit. The slouch is not a strenuous posture, but tends to be forgotten. A device that enforces a slight forward bend in the spine is a string with one end tied to the neck and the other to the… down below.