On the serve, it must be in the service box. Other shots will depend what type of tennis you are playing. In doubles the boundaries are the most outward side lines and the baseline. In singles it is the innermost sidelines and the baseline
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If you are talking about when the ball is hit and where it hits on the court and whether it is in or out, then while playing singles it is that inner long white line. And when playing doubles it is the long outer white line. THose were for the sides and the back horizontal line (baseline) marks the out if it is hit outside (same goes for the other lines)>
on a tennis court there are 2 sets of lines, one considered the doubles alley and the singles alley. in the singles alley, if the ball goes past that line or past the base line without any sign of touching the line then its out. same goes for doubles. but if the ball bounces on the line, or just touches the line its still in. and if the ball hits you even if its going out, the other player still gets the point. so try not to touch it with your body.
A tennis serve is out if it hits the green without hitting the main court, or if it hits the net