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 HIGH ALTITUDE COURTS: Extra duty tennis balls can be played on hard court matches and on indoor and outdoor softer, clay court surfaces as well. Please note that Penn high altitude tennis balls are intended for elevations of at least 4000'. 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.