If a batsman becomes ill or is injured at any time after the players have been nominated for the match, or the umpires are satisfied that there are other good reasons for doing so, he is entitled to have a runner to run for him when his turn comes to bat.
A runner must be a member of the nominated team, must wear similar external equipment to the batsman for whom he's running, must carry a bat and, wherever practicable, should already have batted in that innings. That last point is important because, if the runner were the 'next man in', he would gain an unfair advantage by being able to take a close look at the bowling and the batting conditions before his own turn came to bat. Within the spirit of the game, captains should ensure that this situation does not arise. If it is one of the two opening batsmen who requires the runner then captains should choose one of the later-order batsmen to undertake the duty.
The sole purpose of the runner is to undertake the running for a batsman who cannot do so for himself. A runner, therefore, can only be out Run out (as well as the very rare dismissals for Obstructing the field or Handled the ball), though runners, like all players, are subject to the Laws on unfair play. The batsman with a runner is just as vulnerable to all the methods of dismissal as any other batsman is, but what he needs to remember is that he should not involve himself in running at all.
Forgetting this can prove disastrous. If the batsman with a runner is the non-striker then he is considered as - effectively - out of the game and will usually be directed to stand near the striker's end umpire, his place at the non-striker's end being taken by the runner. The only ways a batsman with a runner can be dismissed when he is not on strike are Obstructing the field, Handled the ball, or if his runner is Run out.
When the batsman with a runner is on strike, the runner will be directed to take up a position where the striker's end umpire can see him. The umpire will usually move across from square leg to the off side and direct the runner to stand on the on side. By doing this the umpire now has a clear view of everybody involved.
The batsman with a runner is only ever in his ground at the wicket-keeper's end. If he is on strike but out of his ground when the wicket is fairly put down, he can be Stumped or Run out, of course. But his runner, too, should take care over where he stands himself since, if he is the one out of his ground when the wicket is fairly put down, he will be Run out and likely to become somewhat unpopular, since the batsman for whom he is running is thereby dismissed!
The real confusion starts when the batsman with a runner hits the ball and, in his excitement, forgets about the runner and starts limping off towards the other end of the pitch! In village cricket it is not unusual to find the batsman who asked for the runner and his runner arriving at the non-striker's end.
If the batsman with a runner now finds himself up at the bowler's end, he is the one who is out Run out if the ball is returned to the wicket-keeper's end and that wicket is broken fairly. Even if the non-striker had completed the run and arrived there before the wicket was broken, it is still the batsman with a runner who should be given out - because he is only in his ground when at the wicket-keeper's end.
If it is the striker, the man with a runner, who is himself out Run out, then no runs are scored and the batsmen are returned to their original ends.