A nipple clamp is a used to stimulate the nipples by applying varying degrees of pressure. They are often used in certain BDSM activities, on both men and women.
Three types of nipple clamp are the clover clamp, tweezer clamp and the common household clothes-pin. Any device that can be used to apply direct pressure can be used.
Weights may be hung from the clamps to increase the pressure on the nipples. This also discourages the clamped person from moving suddenly (for example, jumping when spanked or caned), since this would make the weights swing and increase the pain.
Tweezer clamps consist of two short lengths of metal, no more than 2 to 4 in in length. The ends are curved slightly with small teeth on their ends to improve their ability to grip. Typically, they have a rubber sheath over this end to cover the teeth and to protect the nipple from damage. A small ring wraps around the two pieces of metal and is used to adjust the tension. The user places the two sides of the metal head either side of the nipple and then sliding the ring along the clamp shaft towards the nipple causing the two halves to close tighter together. Tension can be adjusted by vatying the location of the ring; the closer the ring is to the nipple, the tighter the clamp.
The clover clamp, also referred to as a "butterfly clamp", is of Japanese design and has the ability to provide increasing tension if pulled on. Using spring tension, the clamp is applied to the nipple and the tension of its spring holds the clamp in place. This clamp usually provides a very high pain level, and is usually only used by more advanced users. To increase tension on the nipples, small weights such as fishing line sinkers can be attached to the ends of the clamp. As increased pulling tension is applied to the clamp, it will cause the jaws of the clamp to close tighter depending on the amount of pull pressure. Another method is to use the clamps to keep a person in one place. If cord is tied from the clamp to a fixed place, the wearer of the clamps cannot move away because the clamps will tighten as they try to move. Eventually the clamps would be pulled off the nipples, but not before considerable pain and discomfort is felt.
It should be noted that both of the above clamps were originally used in Japan to hold silk for a person doing embroidery work.