The Penn Championship tennis balls offer an extra-duty felt for added durability and optimum playability on demanding surfaces. These USTA (United States Tennis Association) and ITF (International Tennis Federation) approved balls feature a natural rubber core for a consistent feel and reduced shock. The balls are suitable for hard courts and ensure longer wear thanks to an interlocked wool fibre. They are the official balls of USTA leagues and come in a 12-dozen pack (144 balls).