Polyesters. This is the ultimate string for durability, yet it delivers superb power, control and comfort. If you have a fast swing speed and hit the ball hard, then this is your string. German technology refined this already popular tournament polyester string and made it very soft with poly-poly ether material and aluminum fibers. This exotic material makes the string more playable while retaining its durability. A softer string, such as this, is less likely to cause the dreaded "tennis elbow". Big Hitter is a superbly engineered string that holds tension better and lasts longer than standard polyester stings.