This article is from the Table Tennis (Ping Pong) FAQ, by ttennis@bu.edu with numerous contributions by others.
8.1 The ball, having been served or returned in play, shall be struck
so that it passes directly over or around the net and its supports
and touches the opponent's court, except that:
8.1.1 If the ball, having been served or returned in play, returns with
its own impetus over the net it may be struck so that it touches
directly the opponent's court.
8.1.2 If the ball, in passing over or around the net, touches the net or
its supports, it shall be considered to have passed directly.
 
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