AMERICA'S #1 SELLING BALL: The product of 100 years worth of testing and perfecting; Penn Championship is the best selling tennis ball in America.PENN PERFORMANCE: Controlled fiber release provides perfectly even nap, while natural rubber offers consistent feel and reduced shock. The Championship line provides unparalleled consistency every time you hit the court.ALL COURTS: While extra duty tennis balls are best suited for hard court matches, they can also be played on all indoor and outdoor softer, clay court surfaces as well.OFFICIAL BALL: Penn Championship is the Official Ball of USTA Leagues, so you know you're playing with high quality and consistent ball. Available in extra-duty felt, regular duty felt, or extra-duty high altitude felt. USTA and ITF approved.LASTING DURABILITY: Interlocked wool fiber ensures the all courts tennis balls will play like new for as long as possible.