Wilson US Open Extra Duty 4 Tin Pack Sanctioned for ITF and USTA tournaments. The extra duty balls provide optimal performance and durability on clay/indoor courts. Each Tin comes with 3 balls. Since 1979, Wilson has been the only ball that has held the distinction of being the "Official Ball of the U.S. Open". The balls are constructed of premium high-grade wool that is woven into a thicker, denser felt providing superb playability, durability and consistency. Wilson Us Open is made with premium tex/tech industries felt. Extra Duty gives the optimal performance and durability on hard courts. It has High Altitude core for best performance on courts above 3000ft.