Substituted Ball; Wrong Ball
15-1. General
A player must hole out with the ball played from the teeing ground unless the ball is lost, out of bounds or the player substitutes another ball, whether or not substitution is permitted (see Rule 15-2). If a player plays a wrong ball, see Rule 15-3.
15-2. Substituted Ball
A player may substitute a ball when proceeding under a Rule that permits the player to play, drop or place another ball in completing the play of a hole. The substituted ball becomes the ball in play.
If a player substitutes a ball when not permitted to do so under the Rules, that substituted ball is not a wrong ball; it becomes the ball in play. If the mistake is not corrected as provided in Rule 20-6 and the player makes a stroke at a wrongly substituted ball, he incurs the penalty prescribed by the applicable Rule and, in stroke play, must play out the hole with the substituted ball.
15-3. Wrong Ball
a. Match Play
If a player makes a stroke at a wrong ball that is not in a hazard, he loses the hole.
There is no penalty if a player makes a stroke at a wrong ball in a hazard. Any strokes made at a wrong ball in a hazard do not count in the player's score.
If the wrong ball belongs to another player, its owner must place a ball on the spot from which the wrong ball was first played.
If the player and opponent exchange balls during the play of a hole, the first to make a stroke at a wrong ball that is not in a hazard, loses the hole; when this cannot be determined, the hole must be played out with the balls exchanged.
b. Stroke Play
If a competitor makes a stroke or strokes at a wrong ball that is not in a hazard, he incurs a penalty of two strokes.
There is no penalty if a competitor makes a stroke at a wrong ball in a hazard. Any strokes made at a wrong ball in a hazard do not count in the competitor's score.
The competitor must correct his mistake by playing the correct ball or by proceeding under the Rules. If he fails to correct his mistake before making a stroke on the next teeing ground or, in the case of the last hole of the round, fails to declare his intention to correct his mistake before leaving the putting green, he is disqualified.
Strokes made by a competitor with a wrong ball do not count in his score.
If the wrong ball belongs to another competitor, its owner must place a ball on the spot from which the wrong ball was first played.