In tennis, the line umpire 'calls all shots relating to the assigned lines.' The assigned lines can be the center service line, the service line, the baseline or the sideline. Sometimes they make mistakes. They're human after all.
But can you do better? Watch a series of shots and decide whether each was in or out. Some will be traveling upwards of 100 miles per hour and you only get one chance to make the call. Good luck!