Rule 7 of the code (http://assets.usta.com/assets/1/15/2. The Code.4.pdf) states: That last sentence has got me thinking... The trouble is that there are times when this is a bit over simplistic. E.g. when you are receiving a serve that is coming straight to you, even if the ball is 2 inches out you won't see a space between the ball and the line because the ball itself is in the way of that view. The ball would have to be several inches out before you would clearly see space between it and the line. I would say that people can call the ball very accurately - say to within 1/4 inch or less - without clearly seeing space for the reason given above, instead solely relying on seeing the trajectory of the ball and the position of the line. To my understanding, this is how Hawk Eye works also - by seeing the ball's trajectory and not seeing space between the line and ball. On top of that, even if you are looking down the line, if the ball is hit hard enough then it is just a blur and so again seeing space is not at all a reliable method of judging the ball. Seeing the trajectory is in my opinion the best way in this situation also. I think the bit about "99% out is 100% good" is totally correct - just the last sentence needs to re-thought. Something like, "If you wouldn't bet your favourite racket on your out call being correct, don't make it".