Polyesters. This is the ultimate string for durability and it also offers superb power, control, touch and sensitivity. If you have a fast swing speed and hit the ball hard, then this is your string. Big Hitter Blue offers a softer feel than the original yet still offers all the control and durability power players need. German technology refined this already popular tournament polyester string and made it very soft with poly-polyether material. 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". This string can be combined with natural gut. Big Hitter Blue is a superbly engineered string that holds tension better and lasts longer than standard polyester stings.