The first tennis balls were constructed of leather and filled with either human or animal hair. Today's tennis balls use a rubber core that is often pressurized. Before 2002, only unpressurized balls were used, regardless of the playing surface. Since then, manufacturers have begun producing different tennis balls for different playing surfaces, many of which have since been approved for use in tournaments by the International Tennis Federation.
Tennis Court Types
There are four main types of tennis courts: clay, grass, carpet and hard. Clay courts are a mixture of clay, rocks and sand that's packed very tightly, with a looser, softer layer of clay on top. Grass courts use very short, tough grass as the playing surface, much like what's used on a golf green. Hard courts are typically asphalt or concrete, while carpet courts are usually hard courts that are covered with artificial turf. Clay and grass courts are slightly softer than hard and carpet courts.
Hard-Court Tennis Balls
Tennis balls that are constructed for use on hard courts are usually referred to as extra-duty balls. The felt used to cover hard-court tennis balls is usually made from a combination of wool and nylon. Hard-court balls usually use a tighter weave of these materials and feel fuzzier than soft-court tennis balls. This is to add durability, since the ball will be bouncing on a hard, abrasive surface. According Open Tennis, balls with the brand name printed on them in black are made specifically for hard courts.
Soft-Court Tennis Balls
Normally referred to as regular-duty tennis balls, these balls are usually made with regular-duty felt, which is woven more loosely and contains less fuzz than extra-duty felt. Using less fuzzy felt prevents the balls from picking up pieces of dirt or clay from a soft court. Some types -- called slow-speed tennis balls -- are slightly larger and bouncier in order to slow the pace of the ball on fast, soft courts, such as grass. When the manufacturer's name is printed on the ball in red, the ball is to be used on clay.
Pressurized versus Unpressurized
Tennis balls can be purchased with either pressurized or unpressurized cores. Pressurized cores are hollow and filled with air or nitrogen. This provides bounce, but over time, the air or nitrogen escapes, and the ball becomes less bouncy. Pressurized balls with extra-duty felt are the most common, and are designed specifically for use on hard courts. Pressurized balls with regular-duty felt are typically used on soft-grass or clay courts. Unpressurized cores use solid rubber that actually increase in bounciness with use. Unpressurized balls with extra-duty felt are also quite popular and can be used on hard and soft courts.
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