Tennis balls are made out of a hollow rubber core covered by a wool or nylon shell. In addition, the air inside the rubber core is pressurized so that the ball bounces evenly.
Modern tennis balls are made differently compared to 100 years ago. Modern tennis balls are made with a heated press that binds the cloth to the rubber core. This process creates the white markings or seams that are present on every tennis ball that is made. More than a century ago, tennis balls were hand stitched, but this meant that there was not a uniform bounce for every ball. Tennis balls in those days were made completely of rubber and could either be black or white.
In 1972 the International Tennis Federation mandated that all tennis balls were to be yellow in color. The sole reason for this color change was to make it easier for spectators to follow the ball during play.
For professional tennis, each ball is tested to ensure that it meet uniformity requirements. One of the most critical is that a tennis ball bounces between 53 and 58 inches on concrete after being dropped from a height of 100 inches. In addition, each tennis ball must be 2 1/2 inches and weigh exactly 2 ounces.