Consequently, there are different types of tread patterns marketed for different surfaces – smooth soles for play on indoor carpet, pimpled tread for grass and artificial grass courts, and shallow intricate tread patterns for hard and clay courts. The classic pattern is the shallow zigzag or herringbone pattern, but many other designs are also used. Soles may also incorporate small islands of circular design in the tread to create pivot points where torsional or rotational friction is reduced in order to make twisting easier, thus reducing torsional strains on the lower leg.