Made of natural rubber, exercise tubing is available in seven lengths of progessive resistance. Resistance levels, obtained by varying the thickness of the tubing, are color-coded in proprietary Thera-Band colors increasing in difficulty through seven levels from tan to silver. The color-coding system provides positive reinforcement and feedback for gauging results. In general, tan and yellow are used for early rehabilitation or very frail individuals. The most common resistances are red, green & blue. Black, silver and gold are typically used by athletes. Can be used for Pilates exercises!