<P> Tennis balls are filled with air and are surfaced by a uniform felt - covered rubber compound . The felt delays flow separation in the boundary layer which reduces aerodynamic drag and gives the ball better flight properties . Often the balls will have a number on them in addition to the brand name . This helps distinguish one set of balls from another of the same brand on an adjacent court . </P> <P> Tennis balls begin to lose their bounce as soon as the tennis ball can is opened . They can be tested to determine their bounce . Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used; balls intended for use at high altitudes have a lower initial pressure, and inexpensive practice balls are made without internal pressurization . A ball is tested for bounce by dropping it from a height of 254 cm (100 inches) onto concrete; a bounce between 135 and 147 cm (53 and 58 inches) is acceptable--if taking place at sea - level and 20 ° C (68 ° F) with relative humidity of 60%; high - altitude balls have different characteristics when tested at sea - level . </P> <P> The ITF's "Play and Stay" campaign aims to increase tennis participation worldwide, by improving the way starter players are introduced to the game . The ITF recommends a progression that focuses on a range of slower balls and smaller court sizes to introduce the game effectively to both adults and children . The slowest balls, marked with red, or using half red felt, are oversized and unpressurized, or made from foam rubber . The next, in orange, are unpressurized normal sized balls . The last, with green, are half pressured normal sized . </P> <P> Before the development of lawn tennis in the early 1870s, the sport was played as the courtly game of real tennis . England banned the importation of tennis balls, playing cards, dice, and other goods in the Act of Parliament Exportation, Importation, Apparel Act 1463 . In 1480, Louis XI of France forbade the filling of tennis balls with chalk, sand, sawdust, or earth, and stated that they were to be made of good leather, well - stuffed with wool . Other early tennis balls were made by Scottish craftsmen from a wool - wrapped stomach of a sheep or goat and tied with rope . Those recovered from the hammer - beam roof of Westminster Hall during a period of restoration in the 1920s were found to have been manufactured from a combination of putty and human hair, and were dated to the reign of Henry VIII . Other versions, using materials such as animal fur, rope made from animal intestines and muscles, and pine wood, were found in Scottish castles dating back to the 16th century . In the 18th century, 1.9 cm (⁄ in) strips of wool were wound tightly around a nucleus made by rolling a number of strips into a little ball . String was then tied in many directions around the ball and a white cloth covering sewn around the ball . </P>

When were white tennis balls last used at wimbledon