• Tennis ball construction
• Numbers on the tennis balls
• 2000 ITF Approved Tennis Balls
• Fast tennis balls
• Medium tennis balls
• Slow tennis balls
• High altitude tennis balls

Tennis ball construction.

The ball shall have a uniform outer surface. If there are any seams, they are stitchless.
Color
The ball shall be white or yellow.
Size
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,
Weight
The ball can be more than two ounces (56.7 grams) and less than two and one-sixteenth ounces (58.5 grams).
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.

Construction
Almost all tennis balls are hollow, made from inflated rubber covered with fabric. The fabric - called the "nap" or "felt"- is the element that changes the most, largely based on what type of surface the ball is intended. Balls designed for grass courts have different felt than those intended for play on asphalt or clay. It is hardly a disaster to play with the "wrong" ball, but there may be a noticeable performance difference. For example, grass court balls might pick up grit from hardcourt surfaces, which will weigh them down and cause the felt to pill more quickly than it should. Likewise, a regular duty ball may stain if used on grass.

Cans of tennis balls are almost always labeled:

Make sure you get the appropriate type for where you play.

Titanium balls
As with the felt, differences in the formulation of the rubber is the basis for numerous manufacturers' claims of superiority. Again, most players are unlikely to notice this on the court, particularly when the balls are new. However, one recent innovation is worth mentioning. There are now balls made with a specially formulated rubber that is blended with powdered titanium. The metal strengthens the bonds in the core and increases the playable life of the ball. Titanium balls will lose their liveliness over time (once the can is opened) like any other regular tennis ball, but they take longer to wear out on court.

Air inside the ball
Even the air inside the tennis ball can be manipulated - some companies inflate their tennis balls with nitrogen instead of regular air. In lab tests, these balls lose their pressure slower than other balls. But the ultimate long-lasting tennis balls have solid, rubber cores. Often called "pressureless", these balls are a good choice for people who play infrequently, as they do not lose their liveliness over time since there is no air inside them to escape. They do have to be replaced occasionally, however, since the felt will wear out.

" High Altitude" balls
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 ve 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 or

Numbers on the tennis balls

Numbers on the balls are right under the logo.
This is so that when you have, say, 8 adjacent courts and some are using the same brand or type of ball, then there won't be a mix-up of someone's ball sprays onto the other court. Unless, of course, they're both using the same number balls.

The numbers make is easier for you to keep up with your tennis balls. For instance, if you are using Penn 3s, the court on your left has Penn 1s, and the court to your right has Wilson 3s, you can tell at a glance which court to throw the extra ball to and in turn they can get your lost ball to you easier.

The colors are usually black numbers on hard court balls, and red numbers on clay court balls.

2000 International Tennis Federation (ITF) Approved Tennis Balls

The following tennis balls have been approved by the ITF for the calendar year 2000.
The balls fall into four sections:

• Type 1 (fast)
• Type 2 (medium)
• Type 3 (slow)
• High Altitude

Comment: The ITF Technical Commission is currently initiating a review of the Rules of Tennis as they relate to tennis balls and their categorisation.

TYPE 1 TENNIS BALLS - FAST

TYPE 2 TENNIS BALLS - MEDIUM 

TYPE 3 TENNIS BALLS - SLOW

HIGH ALTITUDE TENNIS BALLS

Note 1: This ball is not a pressurised ball. The ball is filled with a micro-cellular material that is designed to replace and simulate the effect of the increased internal pressure of a pressurised ball.