Numbers on the balls

Numbers on the tennis 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 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.