Rules of tennis balls

Tennis balls comprise a hollow rubber core and a wool or nylon shell known as the nap. It must not have stitched seams and must be uniform on the outside. In tournaments they must be white or yellow, but these days they are mostly yellow. They are kept refrigerated for major tournament play.
The rubber core is usually filled with pressurized air so the ball will bounce. The ball must be between 2.5 inches (6.25 cms) and 2.63 inches (6.57cms) in diameter and must weigh at least 56 grammes but no more than 59.4 grammes. The rules of tennis say that the ball must bounce between 53 and 58 inches after being dropped onto concrete from a height of 100 inches. They must have atennis balls forward deformation of more than 0.22 inches and less than 0.29 and a return deformation of more than 0.315 inches and less than 0.425 inches at 18lb load, whatever that means! At altitude, more than 4,000 feet, the ball can be bound more than 48 inches and less than 53 inches.
The internal pressure must be greater than the external pressure. Alternatively, the ball must be bound more than 53 inches and less than 58 inches, but must have an equal internal and external pressure and must have been acclimatized for 60 days or more at the altitude that the tournament is being played. This ball, as opposed to the others, is called a zero-pressure ball or a non-pressurised ball. Balls are changed during tournaments because they begin to lose their bounce after about three hours of play.
At all major tournaments they are changed after the warm up and the first seven games, and after that, every nine games. Studies have shown the fluffier a ball gets, the slower it flies which is why you often see most players check several balls to find the least fluffy before serving. When not in use, tennis balls should be stored in the can they came in. This won't prevent them from losing pressure, but can slow it down.
The ball in play
As soon as the server serves, the ball is in play. It remains in play unless a let or fault is called. In that case it returns to play until the point is finished. The ball is in if it falls on any part of the line. To see how the lines are made you can see the court measurements here. If the ball touches a permanent fixture other than the net, posts, cord or cable, strap or band after it has hit the ground, the player who hit it wins the point. If it hits a permanentt fixture before it touches the ground then the opponent wins the point. If the balls are not changed when they should be, the mistake is corrected when the player who should have served with new balls is next due to serve. Thereafter, the change of balls should follow the correct sequence.