What does the number on a tennis ball mean?

Answer 1

The number on the grip end of a tennis racquet indicates the numerator part of the fractional width of the grip in eighths of an inch. The most common number used is '3', meaning that the grip is 3/8" wide. Other common numbers are '4' for 4/8" wide, and '5' for 5/8" wide. Even though the other sizes (i.e., 1, 2, 6, 7, 8), it is very rare to see them sold in retail stores, as they are "odd" fits, and are, usually, a special-order item, although tennis pro shops and specialty stores can adjust the grip size of virtually any racquet for a modest fee. It should be noted that a few tennis racquet manufacturers use "S", "M", or "L", denoting "small", "medium", and "large", respectively, which might confuse those used to the aforementioned numeric system, but it makes it easier for the manufacturer to sell that product worldwide, especially in places that don't use the American/Imperial system of measurement. Also, in very rare cases, the manufacturer may print the letter or number in a different location (usually on the "throat" (i.e., middle) of the racquet) and/or only on the meant-to-be-disposed-after-purchase grip wrapper which comes with some new racquets. In both cases, the manufacturer believes that makes it is easier for the buyer to see the grip size.

Answer 2

As with Golf, the numbers are used to distinguish your ball(s) from the identical-looking ones being used by other players near you.