In

Landing within the court. A player should call “In!” (against himself) when his opponent's shot has landed so close to a line as to make the decision as to whether the ball was good or not a difficult one to determine. Of course, if the ball was clearly out, even if only the tiniest bit, he should call “Out!” but he should call “In!” and make the gesture indicating the fact (like patting the head of a large dog several times) not only when he's sure the ball was in, but also when he's uncertain about it. That is the tennis code.