By rule, a player has to be certain a ball is out to make an out call. If there is a question as to whether it was in or out, you HAVE to give the point to your opponent---by rule, at least. Not that all players do that. LOL. However, that doesn;t mean a ball has to be out by a foot and leave a glaring mark to be out. If you clearly saw the ball out, you should call it out. If you are playing doubles and you and your partner disagree on the call, you give the benefit of the doubt to your opponents and give the point away. If you are out of position to get a good look at the ball landing, you can certainly defer to your partner if he/she has a certain call. I think the point of the whole thing is to make honest and fair play the standard by which any and all line calls are made. I know for a fact I give away a lot more points than I get on line calls, but I would much rather do that than take points I probably don't deserve. I figure it tends to even out over the long haul, and if I play 2 or 3 out balls and end up losing the match, it was my own fault for allowing it to stay so close as to come down to that.