AMERICA'S #1 SELLING BALL: The result of 100 years worth of testing and perfecting; Penn is the standard by which all other tennis balls are measured. LASTING DURABILITY: Pro Penn Marathon is Penn’s longest lasting tennis ball featuring exclusive Encore Technology for a 22% longer lasting core and high tenacity LongPlay Felt for extended play. 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. CONSISTENT PLAY: Players all over the world choose Penn time and time again for reliable bounce and play not found anywhere else. USTA APPROVED: Penn Tour balls are approved by the PTR so you’re sure to be playing with a high quality and consistent ball.