The rules of cricket
Law 29 (Batsman out of his ground)
1. When out of his ground
A batsman shall be considered to be out of his ground unless his bat or some part of his person is grounded behind the popping crease at that end.
2. Which is a batsman's ground
(a) If only one batsman is within a ground
(i) it is his ground.
(ii) it remains his ground even if he is later joined there by the other batsman.
(b) If both batsmen are in the same ground and one of them subsequently leaves it, (a)(i) above applies.
(c) If there is no batsman in either ground, then each ground belongs to whichever of the batsmen is nearer to it, or, if the batsmen are level, to whichever was nearer to it immediately prior to their drawing level.
(d) If a ground belongs to one batsman then, unless there is a striker with a runner, the other ground belongs to the other batsman irrespective of his position.
(e) When a batsman with a runner is striker, his ground is always that at the wicket-keeper's end. However, (a), (b), (c) and (d) above will still apply, but only to the runner and the non-striker, so that that ground will also belong to either the non-striker or the runner, as the case may be.
3. Position of non-striker
The non-striker, when standing at the bowler's end, should be positioned on the opposite side of the wicket to that from which the ball is being delivered, unless a request to do otherwise is granted by the umpire.