"Butterfly" - nipple clamps are made from metal and are beautifully designed.

Strong hold due to knobbed surface area.
The tension force of the spring is not too hard but will increase when the strings are pulled. Because of the knobbed contact surface the clamps are unlikely to slip off. A very nifty idea!
Not without reason these nipple clamps are one of the most popular on the market. Also suitable for attaching weights.

Length approx 9 centimetres.

Unit 1 pair.