New Feel and Construction: Penn updated the classic Pro Penn Marathon line to address past quality inconsistencies by manufacturing them in a new factory and utilizing a felt-application method similar to US Open ballsPro Penn Marathon is the official ball of PTR because of its guaranteed longevity, durability and visibility. Pro Penn Marathon is available in Extra-Duty Felt (for hard courts), Regular-Duty Felt (for soft courts) and Extra-Duty High Altitude (for higher altitudes). Penn’s Professional tennis balls are the most technologically advanced tennis ball providing player with top of the line core and felt technologies that allow the ball to play at its best for longer periods of time.