HEAD FXP 16 tennis string creates maximum ball pocket for great power and optimum control. A revolutionary string construction features polyester filaments for added durability, control and a crisp feel. Special “Tri-Cluster” FiberGEL filaments are integrated in a Polyamide 6 core. Polyamide 6 Monofilament Wrap with three Polyester fibers inside each Monofilament. DuPont Synthetic 66 Coating.