If you have any doubts as to whether a ball is out or good, you must give your opponent the benefit of the doubt and play the ball as good. You should not play a let.
It is your obligation to call all balls on your side to help your opponent make calls when the opponent requests it, and to call against yourself (with the exception of a first service) any ball that you clearly see out on your opponent's side of the net.
Any "out" or "let" must be made instantaneously (i.e. made before either an opponent has hit the return or the return has gone out of play) otherwise, the ball continues in play.
Do not enlist the aid of spectators in making line calls.
If you call a ball out and then realize it was good, you should correct your call.
To avoid controversy over the score, the Server should announce the set score (e.g. 5-4) before starting a game and the game score (e.g. thirty-forty) prior to serving each point.
If players cannot agree on the score, they may go back to the last score on which there was agreement and resume play from that point or they may spin a racket.
Foot faults are not allowed. If an opponent persists in foot faulting after being warned no to do so, the Referee should be informed.
Do not stall, sulk, complain, or practice gamesmanship.
Wait until the players on another court have completed a point before retrieving or returning a ball.
* Excerpted from the official USTA publication, "The Code," whose principles and guidelines shall apply in any match conducted without officials.
To see the complete Rules of Tennis on USTA.com, click here.