Rule nine?
Rule Number 9
Delivery of Service
(a) In delivering the service, the Server shall stand alternately behind the right and left Courts beginning from the right in every game. If service from a wrong half of the Court occurs and is undetected, all play resulting from such wrong service or services shall stand, but the inaccuracy of station shall be corrected immediately it is discovered.
(b) The ball served shall pass over the net and hit the ground within the Service Court which is diagonally opposite, or upon any line bounding such Court, before the Receiver returns it.