Rule number three?
RULE 3
The Ball
The ball shall have a uniform outer surface and shall be white or yellow in colour. If there are any seams, they shall be stitchless.
The ball shall be more than two and a half inches (6.35cm.) and less than two and five-eighths inches (6.67cm.) in diameter, and more than two ounces (56.7 grams) and less than two and one-sixteenth ounces (58.5 grams) in weight.
The ball shall have a bound of more than 53 inches (135cm.) and less than 58 inches (147cm.) when dropped 100 inches (254cm.) upon a concrete base.
The ball shall have a forward deformation of more than .220 of an inch (.56cm.) and less than .290 of an inch (.74cm.) and a return deformation of more than .315 of an inch (.80cm.) and less than .425 of an inch (1.08cm.) at 18 lb. (8.165kg.) load. The two deformation figures shall be the averages of three individual readings along three axes of the ball and no two individual readings shall differ by more than .030 of an inch (.08cm.) in each case.
For play above 4,000 feet (1219m) in altitude above sea level, two additional types of ball may be used. The first type is identical to those described above except that the bound shall be more than 48 inches (121.92cm) and less than 53 inches (135cm) and the ball shall have an internal pressure that is greater than the external pressure. This type of tennis ball is commonly known as a pressurized ball. The second type is identical to those described above except that they shall have a bound of more than 53 inches (135cm) and less than 58 inches (147cm) and shall have an internal pressure that is approximately equal to the external pressure and have been acclimatized for 60 days or more at the altitude of the specific tournament. This type of tennis ball is commonly known as a zero-pressure or non-pressurized ball.
All tests for bound, size and deformation shall be made in accordance with the Regulations in the Appendix hereto.