The following situation occurred in match play:

The players have played their balls onto the green, mark their balls and pick them up. Player A was closer to the hole than player B. Player B puts his ball back and putts, he goes to his ball to mark it and realises that it was not his ball but his opponent's ball.
Now the confusion was great and none of the players knew who was the first to commit the offence, was it the one who first marked the wrong ball on the green or the one who played the wrong ball.
A referee was called and clarified the situation for the players.

Rule 9.5
This rule only applies if it is known or virtually certain that the opponent (or his caddie) has picked up the player's stationary ball or caused it to move.
If the opponent plays the player's ball as a wrong ball, this is dealt with by rule 6.3c(1) and not by this rule.

In this case, however, Rule 9.5b applies

9.5b Penalty for picking up or deliberately touching the ball or causing the ball to move
If the opponent picks up or deliberately touches the player's stationary ball or causes it to move, the opponent incurs a penalty stroke.

However, there are several exceptions:
Exception 1 - Permission for the opponent to pick up the player's ball: There is no penalty if the opponent picks up the ball because he gives the player a stroke, a hole or the hole play or picks up the player's ball at the player's
player's ball at the player's request.
Exception 2 - Accidentally marking and picking up the player's ball on the green: There is no penalty if the opponent marks the player's ball on the green and inadvertently picks it up thinking it is his own ball.
Exception 3 - Same exceptions as for the player: It is unpunishable if the opponent inadvertently causes the ball to move while acting in accordance with exceptions 2, 3 or 4 of Rule 9.4b.

For marking and picking up the ball by the opponent, exception 2 applies.

But for playing the wrong ball, Rule 6.3c(1) applies
(1) Hitting after the wrong ball. A player may not hit after a faulty ball.
Penalty for playing a wrong ball in breach of Rule 6.3c(1): Basic penalty or loss of hole.


In the meantime, if you would like to know more about the Rules of Golf, please contact me directlyat hans.kienesberger@golf.at.